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FURTHER READING

Bash, R., and Lohr, D. (2001). Yeast chromatin structure and

regulation of GAL gene expression. Prog. Nucl. Acids Res. Mol.

Biol. 65, 197–259.

Johnston, M. (1987). A model fungal gene regulatory mechanism: The

GAL genes of Saccharomyces cerevisiae. Microbiol. Rev. 51,

458–476.

Johnston, M., and Carlson, M. (1992). Regulation of carbon and

phosphate utilization. In The Molecular and Cellular Biology of the

Yeast Saccharomyces Vol. II, pp. 193–281. Cold Springs Harbor

Laboratory Press, Cold Springs Harbor, New York.

Lohr, D. (1997). Nucleosome transactions on the promoters of the

yeast GAL and PHO genes. J. Biol. Chem. 272, 26795–26798.

Lohr, D., Venkov, P., and Zlatanova, J. (1995). Transcriptional

regulation in the yeast GAL gene family: A complex genetic

network. FASEB J. 9, 777–787.

Reece, R., and Platt, A. (1997). Signalling activation and repression of

RNA polymerase II transcription in yeast. BioEssays 19,

1001–1010.

BIOGRAPHY

Dennis Lohr is a Professor in the Department of Chemistry and

Biochemistry at Arizona State University, with principal interests in the

relationships between chromatin structure and gene expression in

eukaryotes. He holds a Ph.D. from the University of North Carolina

(Chapel Hill) and was a postdoctoral associate in the Biochemistry and

Biophysics Department at Oregon State University, in the lab of

K. VanHolde. At Oregon State, he carried out most of the initial

characterizations of yeast chromatin structure and in his own lab

carried out the ﬁrst detailed structural analyses of a single copy

eukaryotic promoter, the GAL1–10 promoter, and its accompanying

gene, GAL1.

Ralph Bash is a postdoctoral Research Associate at the Arizona

Biodesign Institute and the Department of Physics and Astronomy at

Arizona State University with interests in chromatin structure,

chromatin function and molecular imaging. He holds a Masters from

San Jose State University and a Ph.D. from Arizona State University,

where he studied intrinsic DNA bending on GAL genes.

YEAST GAL1–GAL10 SYSTEM 433

Zinc Fingers

Mark Isalan

European Molecular Biology Laboratory (EMBL), Heidelberg, Germany

Zinc ﬁngers are small, compact protein subunits, with

structured peptide chains folding around chelated zinc ions.

Functionally, these subunits carry out a wide variety of tasks

within cells by providing stable structural scaffolds and driving

critical binding interactions, especially between proteins,

DNA, and RNA. Zinc ﬁngers are particularly important in

gene regulation, where many proteins employ them to bind

DNA in a sequence-speciﬁc manner, so as to activate or inhibit

particular genes. Although many families of zinc ﬁngers exist,

each with a characteristic fold or structure, perhaps the best

studied is the “classical” (“Cys

–His

”

or “C2H2”) type.

Classical zinc ﬁngers are extremely versatile, as can be seen by

their abundance in nature: over 700 proteins contain such

domains in the human genome alone, making them the second

most common human protein motif. Zinc ﬁngers may be seen

as convenient molecular building blocks, often used in clusters,

whose modular nature allows them to be conveniently

duplicated, altered, and shufﬂed by evolution. In a biotechnol-

ogy context, zinc ﬁngers may be re-engineered to have novel

binding properties with numerous applications in both

research and medicine.

Zinc Finger Structures

and Families

CLASSICAL ZINC FINGERS

History

The zinc ﬁnger motif was discovered in 1985 by Miller,

McLachlan, and Klug in a protein responsible for

controlling gene expression: the Xenopus transcription

factor IIIA (TFIIIA). Originally, nine novel consecutive

repeats were found, each , 30 amino acids in length.

Since every repeat had a conserved pair of cysteines and

histidines at deﬁned positions, Klug proposed that these

residues could chelate a zinc ion, stabilizing an

independent structural domain, the “zinc ﬁnger.”

Notably, the term “ﬁnger” was derived from the

ﬁnger-like appearance of the sequences when drawn

schematically around a zinc ion, as in Figure 1(A). The

repeats found in TFIIIA are now used as a deﬁnition

of the classical zinc ﬁnger, where individual ﬁngers

conform to the archetypal consensus sequence shown

in Figure 2.

Secondary Structure and Hydrophobic Core

Structural studies revealed that the classical Cys

–His

ﬁnger is a simple and compact unit based on an

antiparallel

-hairpin packed against an

-helix

(Figure 1(B)). In addition to chelating zinc, classical

ﬁngers are stabilized at the core by a hydrophobic cluster

of residues, converging from three ﬁxed positions (see

Figures 1 and 2). Moreover, this hydrophobic triad is

highly conserved, often consisting of tyrosine, phenyl-

alanine, and leucine. By contrast, zinc ﬁngers may

withstand extensive amino acid substitutions at other

positions, without signiﬁcantly altering the overall

structure.

The Role of Zinc

Small protein domains are inherently unstable and, even

though the

bba

fold found in zinc ﬁngers can form

around hydrophobic cores alone, the stability of the fold

is dramatically improved by zinc chelation. Such

stability cannot be obtained using the disulﬁde

bridge – a common extracellular alternative – because

of the highly reducing environment inside cells. Con-

sequently, intracellular proteins have solved the problem

of stabilizing small domains by utilizing metal-binding

sites. Zinc is ideally suited for this purpose as it has a

single oxidation state, a ﬁxed and distinguishable

ionic radius, and can accommodate both nitrogen and

sulfur ligands.

Evolution

Zinc ﬁngers may have evolved originally from peptides

which maintained a simple

bba

structure (similar to that

shown in Figure 1(B)) independently of zinc. In any case,

the

bba

fold is one of the smallest possible units of

protein tertiary structure, and may therefore be one of

the oldest. Zinc ﬁngers are very abundant in nature and

the huge variety found in various organisms, from yeast

to human, may in part be due to their modular nature.

In this regard, not only are zinc ﬁngers independently

folding structural building blocks, but they are often

encoded by single exons. Indeed, exon shufﬂing and gene

duplication may account for why zinc ﬁngers have

spread so widely during evolution.

Genomic Abundance

The recent initial drafts of the human genome have

revealed the extent to which classical zinc ﬁngers are

used by cells: there are 706 genes containing one or more

classical zinc ﬁngers in humans, totaling several thou-

sand ﬁngers, and making them the second most common

human protein family. It is remarkable that only

immunoglobulin domains are more abundant in

humans, and indeed only slightly more so, being found

in 765 genes. By contrast, the Drosophila melanogaster

ﬂy has 357 classical zinc ﬁnger genes, actually making

these the most common protein family in this organism.

Despite the fact that zinc ﬁnger usage varies from

SC:

FIGURE 1 Examples of zinc ﬁnger families and folds. (A) A classical Cys

–His

zinc ﬁnger is drawn schematically as a string of amino acids

(1-letter abbreviations), with cysteine (C) and histidine (H) pairs chelating the central zinc ion. Three circled residues (F, F, L) come together in the

structure to form a hydrophobic core. The ﬁnger shown is Finger 2 from the mouse transcription factor Zif268. (B) Representation of the actual

bba

structure of a classical Cys

–His

zinc ﬁnger. Note the position of the hydrophobic core (h

). (C) A Cys

steroid-receptor zinc ﬁnger from the

estrogen receptor. Note that this protein actually contains two contiguous Cys

zinc ﬁnger repeats, forming a single structural unit. (D) A Cys

–His

zinc ﬁnger from a viral Nucleocapsid protein. (E) A yeast GAL4 domain. (F) The “ring” ﬁnger fold.

436 ZINC FINGERS

organism to organism, these motifs are clearly a wide-

spread feature in all eukaryotic cells.

NON-CLASSICAL ZINC FINGERS

Although zinc-binding proteins have been known for

many years, the usual deﬁnition of zinc ﬁngers maintains

that they are distinct from other zinc-binding protein

subregions by virtue of being autonomously folding,

functional mini-domains. Following the discovery of the

classical Cys

–His

zinc ﬁnger, many other classes of

“zinc ﬁnger” have emerged. Although these form a

diverse collection, often with radically different folding

topologies, these proteins are often referred to in the

literature as zinc ﬁngers. Therefore some well-studied

examples are outlined here below.

Cys

Zinc Fingers

One of the best-characterized nonclassical zinc ﬁngers is

the Cys

type, including the steroid-hormone receptor

family. The estrogen receptor (Figure 1(C)) was the ﬁrst

protein of this type to be elucidated structurally and the

gene contains two Cys

zinc ﬁnger repeats, forming a

single structural unit that is quite unlike the independent

classical zinc ﬁngers of TFIIIA. Functionally, the steroid

receptors form dimers, that bind hormones like estro-

gen, and then mediate activation of gene expression

through sequence-speciﬁc DNA-binding interactions in

the promoter regions of target genes. Further examples

of proteins containing Cys

ﬁnger motifs include other

nuclear hormone receptors, such as the glucocorticoid

and retinoic acid receptors. Many Cys

proteins function

as transcription factors, such as the elongation factor

TFIIS and the adenoviral E1A transactivator.

Cys

–His Zinc Fingers

The Cys

–His zinc ﬁnger is another variant deﬁned by

its characteristic combination of cysteine and histidine

residues. For example, the nucleocapsid protein, found

in certain viruses such as HIV, contains Cys

–His

ﬁngers, with various nucleic acid binding properties

that facilitate hybridization and the viral life-cycle

(Figure 1(D)). Other examples of Cys

–His proteins

include MetRS and the ﬁrst ﬁnger in the LIM domain.

Multi-Zinc Proteins

Moving beyond the simpler zinc ﬁnger domains, which

fold around a single zinc ion, there exist larger more

complex folds, each employing two or more zinc ions.

Examples include the yeast GAL4 domain (Figure 1(E))

and the “ring” ﬁnger (Figure 1(F)). Note that although

these larger domains are not quite as prevalent as the

single-zinc modules, they nonetheless form quite abun-

dant families. For example, the “ring” ﬁnger family

ranks as the 15th most abundant domain in the human

genome, with 210 members, while another fold, the

“PHD” ﬁnger, ranks 34th with 84 members.

Zinc Finger Functions

INTERACTIONS WITH DNA AND RNA

Since their discovery, zinc ﬁngers have been associated

with binding nucleic acids, namely, DNA and RNA.

Indeed, the archetypal zinc ﬁnger protein, TFIIIA, is

bi-functional, having distinct modes of binding which

allow it to bind both to the DNA internal control region

of 5S RNA genes and to the transcribed RNA gene

product. In this sense, zinc ﬁngers are master gene

regulators, with the potential to inﬂuence both tran-

scription and translation.

Structure of Zinc Finger Bound to DNA

The majority of our knowledge of zinc ﬁnger functions

has focused on their ability to bind speciﬁc DNA

sequences. The ﬁrst structure of zinc ﬁngers bound to

DNA, that of the DNA-binding domain of the mouse

transcription factor Zif268, was solved by Carl Pabo

and colleagues in 1991. The structure revealed that each

of the three classical zinc ﬁngers occupies the major

groove, wrapping around the DNA for almost one turn

of the double helix (Figure 3A). Base speciﬁc contacts

occur primarily from consecutive turns of the zinc ﬁnger

-helices to the guanine-rich strand of the DNA.

DNA Recognition Helix

Essentially, classical zinc ﬁngers use the

-helix as a

“reading head” to recognize speciﬁc DNA sequences.

Contacts from amino acids consist primarily of electro-

static interactions and hydrogen bonds, although

hydrophobic interactions also play a role. Convention-

ally, the amino acids in the

-helix are numbered

FIGURE 2 Zinc ﬁnger amino acid sequences. (A) Consensus

sequence of a classical Cys

–His

zinc ﬁnger, where X is any amino

acid and

represents hydrophobic residues (typically Y, F, and L).

Zinc-chelating cysteine and histidine residues are circled.

-helical

positions (numbered relative to position þ 1) are indicated above the

sequence. (B) The sequence of Finger 2 of Zif268, showing conformity

with the consensus sequence.

ZINC FINGERS 437

relative to a deﬁned ﬁrst position, Position þ 1

(see Figures 2 and 3(B)). Since positions 2 1, þ 3, and

þ6 are all to be found on the external face of the helix,

these can form a binding surface that is especially useful

in DNA recognition because the spacing of the residues

matches the register of the base pairs. It should be noted,

however, that contacts from other positions, especially

position þ2, are also important in DNA recognition and

that not all zinc ﬁnger–DNA interactions are as regular

as the canonical structure of Zif268.

DNA Recognition “Code”

The most striking feature of the canonical Zif268

structure is the presence of regularly spaced one-to-one

contacts between amino acids and DNA bases, from the

same ﬁxed helical positions in each ﬁnger (Figure 3B). By

studying mutations at these positions, it has emerged

that classical zinc ﬁngers are a special case in terms of

protein–DNA interactions. Unlike for most DNA-

binding proteins, these positions do seem to ﬁt a

residue-base code-like pattern. Positions 2 1, þ 2, þ 3,

and þ 6inthe

-helix are particularly important

(Figure 3B), and some major residue-base associations

are outlined in Figure 3C.

As a note of caution, it is important to mention that

the code is not absolute, and is certainly an over-

simpliﬁcation of the interplay between different residues

in the protein and between factors in the DNA structure,

such as base stacking and unwinding. Nonetheless,

thinking about zinc ﬁnger DNA interactions in this way

can give a valuable approximation into the possible

binding modes of a given zinc ﬁnger protein.

RNA Recognition

Unlike the double helical scaffold of DNA, RNA has

much more complex folds, with every RNA presenting a

unique binding surface. Nonetheless, zinc ﬁngers have

the versatility to accommodate such structures, with

contacts from all over the ﬁnger framework. Like DNA

recognition, electrostatic interactions and hydrogen

bonding play a major role, with amino acids

contacting either RNA bases or the phosphodiester

backbone. Note that in RNA recognition there is a

greater potential for hydrophobic interactions, with

aromatic residues such as Tyr, Phe, and Trp stacking

around extruding bases.

NON-NUCLEIC ACID INTERACTIONS

Protein–Protein Interactions

Zinc ﬁngers are not limited to controlling gene

expression, but also play an important cellular role in

mediating speciﬁc protein–protein interactions.

ACGCCCA G

GC GGTGCGG

Glu 3

His 3

Thr 6

Glu 3

Arg 6

Arg-1

Asp2 Asp2

Asp2

3´4´5´6´7´8´9´10´

5678910

11´

F1 F2 F3

3´

5´

3´

Position in Zn finger

-1

ACDEFGH KLMNPQRSTVWY

Base specified

FIGURE 3 Interactions between zinc ﬁngers and DNA. (A) Side view

of Zif268 binding DNA (Pavletich and Pabo (1991). Zinc ﬁnger-DNA

recognition: crystal structure of a Zif268-DNA complex at 2.1A

Science 252, 809–817. The three zinc ﬁngers (F1–F3) bind the major

groove of the DNA. (B) Schematic diagram of a simple model of

recognition between the three zinc ﬁngers of Zif268 and the triplet

subsites of an optimized DNA-binding site. Arrows indicate contacts

by recognition residues with bases in each DNA strand. Note that most

contacts are to the G-rich strand. Since the N-terminal ﬁnger contacts

the 3

end of the DNA and the C-terminal ﬁnger the 5

end, binding is

said to be antiparallel. (C) Zinc ﬁnger DNA-recognition code. The grid

summarizes common associations between amino acid residues (A–Y)

and the bases they face in a classical zinc ﬁnger DNA-binding site.

Darker shading of boxes indicates stronger associations. Note that

contacts from overlapping C-terminal zinc ﬁngers (especially from

position 2) can signiﬁcantly alter the coding speciﬁcity shown here

for position þ 6.

438 ZINC FINGERS

Whereas nucleic acid recognition usually requires

clusters of ﬁngers, cytoplasmic proteins often contain

only one or two ﬁngers. This is possibly because a single

protein-binding ﬁnger has sufﬁcient afﬁnity to bind

effectively, given the more variable binding surface

presented by other proteins.

Lipid Binding

Recently, the known binding repertoire of zinc ﬁngers

has been extended to include lipid recognition. This is

exempliﬁed by the FYVE zinc ﬁnger family, that is found

in a wide range of eukaryotic cells. These nonclassical

ﬁngers have a complex structure consisting of multiple

loops, two twin-stranded

-sheets and an

-helix

stabilized by a pair of zinc ions. Functionally, the

domains bind to phosphatidylinositol 3-phosphate

(PI3P) in cellular membranes, commonly found on

endosomes, and mediate further recruitment of a range

of proteins.

ZINC FINGERS IN BIOTECHNOLOGY

Classical zinc ﬁngers are the ideal scaffold for protein

engineering. They are small and versatile, withstanding

extensive mutations without destroying their basic

structure. Moreover, individual ﬁnger modules may be

linked in tandem repeats to allow recognition of

extended target surfaces such as DNA grooves. Novel

DNA-binding zinc ﬁngers have been routinely engin-

eered for nearly a decade and are beginning to

demonstrate their utility in biotechnical applications.

By fusing with suitable effector domains, effective

transcription activators and repressors can be syn-

thesized that recognize speciﬁc DNA sequences, typi-

cally 9–18 bp long. These constructs can up-regulate or

down-regulate speciﬁc genes in a cell and have been used

in a wide variety of contexts, including as antiviral

agents and angiogenesis-promoting factors.

FURTHER READING

Beerli, R. R., and Barbas, C. F., III (2002). Engineering polydactyl zinc-

ﬁnger transcription factors. Nat. Biotechnol. 20, 135– 141.

Berg, J. M., and Shi, Y. (1996). The galvanization of biology: A

growing appreciation for the roles of zinc. Science 271,

1081–1085.

Choo, Y., and Isalan, M. (2000). Advances in zinc ﬁnger engineering.

Curr. Opin. Struct. Biol. 10, 411– 416.

Klug, A., and Schwabe, J. W. (1995). Protein motifs 5. Zinc ﬁngers.

FASEB J. 9, 597–604.

Lander, E. S., et al. (2001). Initial sequencing and analysis of the human

genome. Nature 409, 860–921.

Mackay, J. P., and Crossley, M. (1998). Zinc ﬁngers are sticking

together. Trends Biochem. Sci. 23,1–4.

Pabo, C. O., Peisach, E., and Grant, R. A. (2001). Design and selection

of novel Cys

His

zinc ﬁnger proteins. Annu. Rev. Biochem. 70,

313–340.

BIOGRAPHY

Mark Isalan is a fellow of the Wellcome Trust, UK, and is currently at

the European Molecular Biology Laboratory (EMBL), Heidelberg. His

principal research interests include combinatorial and design

approaches to protein engineering, gene regulation, and gene net-

works. He holds a Ph.D. from the University of Cambridge, where he

trained at the MRC Laboratory of Molecular Biology, developing

methods to engineer novel DNA-binding zinc ﬁngers.

ZINC FINGERS 439

Editors-in-Chief

William J. Lennarz

State University of New York at Stony Brook, Stony Brook,

New York, USA

Section: Lipids, Carbohydrates, Membranes and Membrane Proteins

WILLIAM J. LENNARZ received his B.S. in Chemistry

from Pennsylvania State University and a Ph.D. in

Organic Chemistry from the University of Illinois.

Subsequently he carried out postdoctoral work at

Harvard with Konrad Bloch on fatty acid biosynthesis.

In 1962 he was appointed Assistant Professor at Johns

Hopkins in the Department of Physiological Chemistry.

After promotion to Associate Professor in 1967, and full

Professor in 1971, he remained at Hopkins until 1983. At

that time, he was appointed Robert A. Welch Professor

and Chair of the Department of Biochemistry and

Molecular Biology at the University of Texas Cancer

Center, M.D. Anderson Hospital. In 1989 he became a

Leading Professor and Chair of the Department of

Biochemistry and Cell Biology at SUNY at Stony

Brook. In 1990 he founded and became Director of the

Institute for Cell and Developmental Biology at Stony

Brook.

Dr. Lennarz has served on many national and

international committees. He has served as President

of the Biochemistry Chairman’s Organization, President

of the American Society for Biochemistry and Molecular

Biology and President of the Society for Glycobiology.

He was a member of the Executive Committee of the

International Union of Biochemistry and Molecular

Biology for almost a decade.

He has presented special lectures at the University of

Notre Dame, the NIH, the University of West Virginia,

Johns Hopkins University, Florida State University, the

University of California at San Diego, the University of

Arkansas, Indiana University and the Medical College

of Virginia.

He is a member of the National Academy of Sciences.

The focus of his early work was on lipids and bacterial

cell surfaces. More recent efforts have been in the

structure, biosynthesis and function of cell surface

glycoproteins. The biosynthesis studies initially were

carried out in liver and oviduct, but these efforts now are

focused in yeast. The functional studies have concen-

trated on the role of cell surface glycoproteins in

fertilization and early development in the sea urchin

and, more recently, the frog. For over 30 years Dr.

Lennarz’ research has been supported by federal sources,

primarily the National Institutes of Health. Recently he

was appointed Distinguished Professor and Chair of his

department.

M. Daniel Lane

The Johns Hopkins University, School of Medicine, Baltimore,

Maryland, USA

Section: Metabolism, Vitamins and Hormones

M. DANIEL LANE received B.S. and M.S. degrees in

1951 and 1953 from Iowa State University and a Ph.D.

in 1956 from the University of Illinois. He was a Senior

Postdoctoral Fellow with Professor Feodor Lynen at the

Max-Planck Institute Fur Zellchemie in Munich.

Following faculty positions at Virginia Polytechnic

Institute and New York University School of Medicine,

he joined the faculty at the Johns Hopkins University

School of Medicine in 1969 and served as DeLamar

Professor and Director of the Department of Biological

Chemistry from 1978 to 1997. He is presently Distin-

guished Service Professor at Johns Hopkins. In 2002 he

received an honorary degree, Doctor of Humane Letters,

from Iowa State University.

Dr. Lane was elected to membership in the National

Academy of Sciences (in 1987) and was elected as a

Fellow of the American Academy of Arts and Sciences

(in 1982) and of the American Society of Nutritional

Sciences (in 1996). He received the Mead Johnson

Award from the American Society for Nutritional

Sciences in 1966 for his research on biotin-dependent

enzymes and in 1981, the William C. Rose Award from

the American Society for Biochemistry and Molecular

Biology for his work on the insulin receptor. In

1990–1991 Lane served as President of the American

Society of Biochemistry and Molecular Biology. He has

presented many named lectureships (including the

Feodor Lynen Lecture in Germany in 1999) and served

on numerous editorial boards including the Journal of

Biological Chemistry and the Annual Reviews of

Biochemistry. Currently he is Associate Editor for

Biochemical and Biophysical Research Communi-

cations.

Dr. Lane has published 280 research papers in major

scientiﬁc journals. His early work focused on various

enzymatic CO

ﬁxation reactions, notably the mechan-

isms by which the B-vitamin, biotin, functions in

enzymes to catalyze carboxylation. Dr. Lane’s work on

the regulation of acetyl-CoA carboxylase, the key

regulatory enzyme of fatty acid synthesis, led him to

his present interests which are to understand the basic

mechanisms of lipogenesis, adipogenesis and the con-

sequence of aberrations in these processes, most notably

obesity. Research currently underway in his laboratory

focuses on: (1) the genes that signal stem cell “commit-

ment” to the adipocyte lineage and subsequent differ-

entiation into adipocytes, and (2) the mechanisms by

which the region of the brain, known as the hypothala-

mus, monitors and controls the drive to eat.

EDITORS-IN-CHIEF

Associate Editors

Ernesto Carafoli

Universita

degli Studi di Padova, Padova, Italy

Section: Bioenergetics

ERNESTO CARAFOLI earned his M.D. degree at the

University of Modena in Italy in 1957. After postdoc-

toral studies in the Laboratory of Albert L. Lehninger at

Johns Hopkins University in the mid 1960s he returned

to his home institution in Italy where he worked until

1973, when he was appointed Professor of Biochemistry

at the Swiss Federal Institute of Technology (ETH) in

Zurich. He returned to Italy in 1998 as a Professor of

Biochemistry at the University of Padova, where he now

also directs the newly founded Venetian Institute of

Molecular Medicine (VIMM).

Dr. Carafoli became interested in calcium as a

signaling agent during his post-doctoral days at Johns

Hopkins. When he arrived there his main interests were

in mitochondrial bioenergetics and it was thus natural

for him to expand them to the newly discovered area of

mitochondrial calcium transport. He was involved in

most of the early discoveries in the ﬁeld, and he

continued to work on mitochondria and calcium after

his return to Italy and until he moved to the ETH. There

his interests still remained focused on calcium, but the

emphasis shifted to the proteins that transport it across

membranes and to those that process its signal. His

favorite object of study became the calcium pumps,

especially that of the plasma membrane, an enzyme

which is essential to the regulation of calcium homeo-

stasis and thus to the well being of cells. His

contributions on the enzyme, especially after he puriﬁed

it in 1979, have helped establishing most of its properties

and have clariﬁed important problems of mechanism,

regulation and structure.

Dr. Carafoli has authored or co-authored about 450

peer-reviewed articles and reviews, and has edited or

co-edited about 20 books. He has served on the Editorial

or Advisory Boards of several periodicals and has

organized about 30 International Workshops and

Symposia. He has been featured as a plenary or

honorary lecturer at numerous events ranging from

specialized Workshops to International Symposia and

Congresses. Dr. Carafoli’s honors and awards include

several international prizes and medals, memberships in

several Academies, and three honorary degrees.

Don W. Cleveland

University of California, San Diego, La Jolla, CA, USA

Section: Cell Architecture and Function

DON W. CLEVELAND has been a longstanding

contributor to the elucidation of regulation of assembly

of mitotic spindles and chromosome movement and

how errors in these contribute to the chromosome loss

characteristic of human tumors. He discovered the

tubulin gene families encoding the major subunits of

microtubules and the ﬁrst mammalian example of

control of gene expression through regulated RNA

instability. He identiﬁed components required for

microtubule nucleation and anchoring during

spindle assembly. He identiﬁed the ﬁrst human centro-

meric protein (CENP-B). He then discovered CENP-E,

the centromere-associated, microtubule-motor that he

showed to be essential for chromosome attachment and

for activation and silencing of the mitotic checkpoint, the

cell cycle control mechanism that prevents errors of

chromosome segregation in mitosis.

Dr. Cleveland has also been a leading force in

dissecting the disease mechanism for major human

neurodegenerative disorders. He initially puriﬁed and

characterized tau, the microtubule-associated protein

that assembles aberrantly in human dementias includ-

ing Alzheimer’s disease and Pick’s disease. He

established that the extreme asymmetry of neurons

acquired during development is achieved with a

deformable array of interlinked neuroﬁlaments, micro-

tubules and actin. He showed that disorganization of

neuroﬁlament arrays caused selective death of motor

neurons in mice and humans. He also demonstrated

that neuronal death could also arise by a toxicity of

mutant superoxide dismutase unrelated to its normal

activity, thereby uncovering the mechanism underlying

the major genetic form of amyotrophic lateral

sclerosis. He showed that this toxicity could be

sharply ameliorated by lowering the content of

neuroﬁlaments.

Dr. Cleveland is currently Head, Laboratory for Cell

Biology in the Ludwig Institute for Cancer Research and

Professor of Medicine, Neurosciences and Cellular and

Molecular Medicine at the University of California at

San Diego. He is also the Editor of the Journal of Cell

Biology and Current Opinion in Cell Biology.

Jack E. Dixon

University of California, San Diego School of Medicine,

La Jolla, CA, USA

Section: Protein/Enzyme Structure, Function, and Degradation

JACK E. DIXON earned his Ph.D. in Chemistry at the

University of California, Santa Barbara in 1971 and did

his postdoctoral training in Biochemistry at the Univer-

sity of California, San Diego.

Dr. Dixon is a pioneer and leader in the structure and

function of the protein tyrosine phosphatases (PTPases).

He demonstrated that the unique catalytic mechanism of

the PTPases proceeds via a novel cysteine-phosphate

intermediate. He discovered the ﬁrst dual-speciﬁcity

phosphatase, which led to the identiﬁcation of the cell

cycle protein, p80

cdc25

, as a phosphatase. He also

showed that the bacteria responsible for the plague or

“black death” harbor the most active PTPase ever

described. He and his colleagues went on to demonstrate

that this PTPase gene product is essential for the

pathogenesis of the bacteria. Dr. Dixon and his

colleagues determined X-ray structures for both tyrosine

and dual speciﬁcity phosphatases. Dr. Dixon also found

that sequences outside of the PTPase catalytic domain

could function to direct the subcellular localization of

the PTPases and to restrict their substrate speciﬁcity.

This is now a widely acknowledged regulatory paradigm

for the PTPases. Recently, his laboratory demonstrated

that the tumor suppressor gene, PTEN, which

shares sequence identity with the PTPases, catalyzes

the dephosphorylation of a lipid second messenger,

phosphatidylinositol 3,4,5-trisphosphate (PIP3). This

represents the ﬁrst example of a PTPase dephosphor-

ylating a lipid second messenger. PIP3 activates the

protein kinase, AKT, which plays a critical role in

controlling the balance between apoptosis and cell

survival. The loss of the PTEN gene elevates PIP3 levels

leading to constitutive activation by AKT and oncogen-

esis. Recently, Dr. Dixon in collaboration with Nikola

Pavletich determined the X-ray structure of PTEN. Their

structure– function studies explain the PIP3 substrate

speciﬁcity of PTEN and also provide a rationale for

many of the mutations seen in human cancers. Earlier in

his career, Dr. Dixon adopted the tools of molecular

biology as they became available in the 1970s, and his

laboratory was among the ﬁrst to use synthetic

oligonucleotides to isolate and extensively characterize

cDNAs encoding peptide hormones.

Dr. Dixon is Professor of Pharmacology, Cellular and

Molecular Medicine and Chemistry and Biochemistry

and Dean of Scientiﬁc Affairs at the University of

California, San Diego. He is a member of the National

Academy of Sciences, the Institute of Medicine and the

American Academy of Arts and Sciences. Dr. Dixon was

the recipient of the 2003 William C. Rose Award from

the American Society for Biochemistry and Molecular

Biology.

John H. Exton

Howard Hughes Medical Institute, Vanderbilt University School of

Medicine, Nashvillie, TN, USA

Section: Signaling

JOHN H. EXTON was born and educated in New

Zealand where he received his medical training and a

Ph.D. in Biochemistry from the University of Otago in

1963. He did postdoctoral work at Vanderbilt Univer-

sity under Charles R. Park and Earl W. Sutherland, and

became an Investigator of the Howard Hughes Medical

Institute in 1968 and Professor of Physiology in 1970.

He is presently Professor of Molecular Physiology and

Biophysics, Professor of Pharmacology and a Hughes

Investigator at Vanderbilt.

Dr. Exton’s research initially focused on the changes

in carbohydrate metabolism in liver during diabetes and

treatment with various hormones using the perfused rat

liver as the experimental system. His work concen-

trated on gluconeogenesis and identiﬁed the enzymatic

reactions that were under control by insulin, epineph-

rine (adrenaline), glucagon and glucocorticoids, and

demonstrated the importance of cyclic AMP in the

regulation of these reactions. The role played by the

supply of substrates, especially of alanine, was also

shown.

Dr. Exton then turned his attention to the action of

epinephrine (adrenaline) and demonstrated that many of

its actions were not mediated by cyclic AMP but by

calcium ions. This led to study of the breakdown of

inositol phospholipids by phospholipase C that underlay

the increase in calcium. Later this resulted in the

discovery of G

, a novel G protein that activated

phospholipase C. Further studies demonstrated that

agonists caused the breakdown of another phospholipid

(phosphatidylcholine) by another phospholipase (phos-

pholipase D). Current work is focused on the physio-

logical role of phospholipase D.

Dr. Exton has authored over 350 scientiﬁc articles

and is presently an Associate Editor of the Journal of

Biological Chemistry. He has served on many scientiﬁc

review groups and as a reviewer for many journals. He

has won numerous awards, most notably the Lilly

ASSOCIATE EDITORS