A novel MHC class I-like gene is mutated in patients with hereditary haemochromatosis

Hereditary haemochromatosis (HH), which affects some 1 in 400 and has an estimated carrier frequency of 1 in 10 individuals of Northern European descent, results in multi-organ dysfunction caused by increased iron deposition, and is treatable if detected early. Using linkage-disequilibrium and full haplotype analysis, we have identified a 250-kilobase region more than 3 megabases telomeric of the major histocompatibility complex (MHC) that is identical-by-descent in 85% of patient chromosomes. Within this region, we have identified a gene related to the MHC class I family, termed HLA-H, containing two missense alterations. One of these is predicted to inactivate this class of proteins and was found homozygous in 83% of 178 patients. A role of this gene in haemochromatosis is supported by the frequency and nature of the major mutation and prior studies implicating MHC class I-like proteins in iron metabolism.

The cysteine switch: a principle of regulation of metalloproteinase activity with potential applicability to the entire matrix metalloproteinase gene family

The general applicability of the "cysteine-switch" activation mechanism to the members of the matrix metalloproteinase (MMP) gene family is examined here. All currently known members of the MMP gene family share the characteristic that they are synthesized in a latent, inactive, form. Recent evidence suggests that this latency in human fibroblast collagenase (HFC) is the result of formation of an intramolecular complex between the single cysteine residue in its propeptide domain and the essential zinc atom in the catalytic domain, a complex that blocks the active site. Latent HFC can be activated by multiple means, all of which effect the dissociation of the cysteine residue from the complex. This is referred to as the "cysteine-switch" mechanism of activation. The propeptide domain that contains the critical cysteine residue and the catalytic domain that contains the zinc-binding site are the only two domains common to all of the MMPs. The amino acid sequences surrounding both the critical cysteine residue and a region of the protein chains containing two of the putative histidine zinc-binding ligands are highly conserved in all of the MMPs. A survey of the literature shows that many of the individual MMPs can be activated by the multiple means observed for latent HFC. These observations support the view that the cysteine-switch mechanism is applicable to all members of this gene family. This mechanism is unprecedented in enzymology as far as we know and offers the opportunity for multiple modes of physiological activation of these important enzymes. Since conditions in different cells and tissues may match those necessary to effect one of these activation modes for a given MMP, this may offer metabolic flexibility in the control of MMP activation.

The CONSTANS gene of Arabidopsis promotes flowering and encodes a protein showing similarities to zinc finger transcription factors

The vegetative and reproductive (flowering) phases of Arabidopsis development are clearly separated. The onset of flowering is promoted by long photoperiods, but the constans (co) mutant flowers later than wild type under these conditions. The CO gene was isolated, and two zinc fingers that show a similar spacing of cysteines, but little direct homology, to members of the GATA1 family were identified in the amino acid sequence. co mutations were shown to affect amino acids that are conserved in both fingers. Some transgenic plants containing extra copies of CO flowered earlier than wild type, suggesting that CO activity is limiting on flowering time. Double mutants were constructed containing co and mutations affecting gibberellic acid responses, meristem identity, or phytochrome function, and their phenotypes suggested a model for the role of CO in promoting flowering.

Leptomycin B inactivates CRM1/exportin 1 by covalent modification at a cysteine residue in the central conserved region

The cellular target of leptomycin B (LMB), a nuclear export inhibitor, has been identified as CRM1 (exportin 1), an evolutionarily conserved receptor for the nuclear export signal of proteins. However, the mechanism by which LMB inhibits CRM1 still remains unclear. CRM1 in a Schizosaccharomyces pombe mutant showing extremely high resistance to LMB had a single amino acid replacement at Cys-529 with Ser. The mutant gene, named crm1-K1, conferred LMB resistance on wild-type S. pombe, and Crm1-K1 no longer bound biotinylated LMB. (1)H NMR analysis showed that LMB bound N-acetyl-L-cysteine methyl ester through a Michael-type addition, consistent with the idea that LMB binds covalently via its alpha, beta-unsaturated delta-lactone to the sulfhydryl group of Cys-529. When HeLa cells were cultured with biotinylated LMB, the only cellular protein bound covalently was CRM1. Inhibition by N-ethylmaleimide (NEM), an alkylating agent, of CRM1-mediated nuclear export probably was caused by covalent binding of the electrophilic structure in NEM to the sulfhydryl group of Cys-529, because the crm1-K1 mutant showed the normal rate for the export of Rev nuclear export signal-bearing proteins in the presence of not only LMB but also NEM. These results show that the single cysteine residue determines LMB sensitivity and is selectively alkylated by LMB, leading to CRM1 inactivation.

Rapid method for the determination of 5-hydroxytryptamine and 5-hydroxyindoleacetic acid in small regions of rat brain

A rapid and sensitive method for measuring 5-hydroxytryptamine and 5-hydroxyindoleacetic acid, using o-phthalaldehyde and L-cysteine, is presented, enabling both compounds to be measured in small areas of rat brain.

Methionine residues as endogenous antioxidants in proteins

Cysteine and methionine are the two sulfur-containing residues normally found in proteins. Cysteine residues function in the catalytic cycle of many enzymes, and they can form disulfide bonds that contribute to protein structure. In contrast, the specific functions of methionine residues are not known. We propose that methionine residues constitute an important antioxidant defense mechanism. A variety of oxidants react readily with methionine to form methionine sulfoxide, and surface exposed methionine residues create an extremely high concentration of reactant, available as an efficient oxidant scavenger. Reduction back to methionine by methionine sulfoxide reductases would allow the antioxidant system to function catalytically. The effect of hydrogen peroxide exposure upon glutamine synthetase from Escherichia coli was studied as an in vitro model system. Eight of the 16 methionine residues could be oxidized with little effect on catalytic activity of the enzyme. The oxidizable methionine residues were found to be relatively surface exposed, whereas the intact residues were generally buried within the core of the protein. Furthermore, the susceptible residues were physically arranged in an array that guarded the entrance to the active site.

Expression and structure of the human NGF receptor

The nucleotide sequence for the human nerve growth factor (NGF) receptor has been determined. The 3.8 kb receptor mRNA encodes a 427 amino acid protein containing a 28 amino acid signal peptide, an extracellular domain containing four 40 amino acid repeats with six cysteine residues at conserved positions followed by a serine/threonine-rich region, a single transmembrane domain, and a 155 amino acid cytoplasmic domain. The sequence of the extracellular domain of the NGF receptor predicts a highly ordered structure containing a negatively charged region that may serve as the ligand-binding site. This domain is conserved through evolution. Transfection of a full-length cDNA in mouse fibroblasts results in stable expression of NGF receptors that are recognized by monoclonal antibodies to the human NGF receptor and that bind [125I]NGF.

A thiol peroxidase is an H2O2 receptor and redox-transducer in gene activation

The Yap1 transcription factor regulates hydroperoxide homeostasis in S. cerevisiae. Yap1 is activated by oxidation when hydroperoxide levels increase. We show that Yap1 is not directly oxidized by hydroperoxide. We identified the glutathione peroxidase (GPx)-like enzyme Gpx3 as a second component of the pathway, serving the role of sensor and transducer of the hydroperoxide signal to Yap1. When oxidized by H2O2, Gpx3 Cys36 bridges Yap1 Cys598 by a disulfide bond. This intermolecular disulfide bond is then resolved into a Yap1 intramolecular disulfide bond, the activated form of the regulator. Thioredoxin turns off the pathway by reducing both sensor and regulator. These data reveal a redox-signaling function for a GPx-like enzyme and elucidate a eukaryotic hydroperoxide-sensing mechanism. Gpx3 is thus a hydroperoxide receptor and redox-transducer.

A family of proteins structurally and functionally related to the E6-AP ubiquitin-protein ligase

E6-AP is a 100-kDa cellular protein that interacts with the E6 protein of the cancer-associated human papillomavirus types 16 and 18. The E6/E6-AP complex binds to and targets the p53 tumor-suppressor protein for ubiquitin-mediated proteolysis. E6-AP is an E3 ubiquitin-protein ligase which accepts ubiquitin from an E2 ubiquitin-conjugating enzyme in the form of a thioester and then directly transfers the ubiquitin to targeted substrates. The amino acid sequence of E6-AP shows similarity to a number of protein sequences over an approximately 350-aa region corresponding to the carboxyl termini of both E6-AP and the E6-AP-related proteins. Of particular note is a conserved cysteine residue within the last 32-34 aa, which in E6-AP is likely to be the site of ubiquitin thioester formation. Two of the E6-AP-related proteins, a rat 100-kDa protein and a yeast 95-kDa protein (RSP5), both of previously unknown function, are shown here to form thioesters with ubiquitin. Mutation of the conserved cysteine residue of these proteins destroys their ability to accept ubiquitin. These data strongly suggest that the rat 100-kDa protein and RSP5, as well as the other E6-AP-related proteins, belong to a class of functionally related E3 ubiquitin-protein ligases, defined by a domain homologous to the E6-AP carboxyl terminus (hect domain).

Nuclear localization of the C2H2 zinc finger protein Msn2p is regulated by stress and protein kinase A activity

Msn2p and the partially redundant factor Msn4p are key regulators of stress-responsive gene expression in Saccharomyces cerevisiae. They are required for the transcription of a number of genes coding for proteins with stress-protective functions. Both Msn2p and Msn4p are Cys2His2 zinc finger proteins and bind to the stress response element (STRE). In vivo footprinting studies show that the occupation of STREs is enhanced in stressed cells and dependent on the presence of Msn2p and Msn4p. Both factors accumulate in the nucleus under stress conditions, such as heat shock, osmotic stress, carbon-source starvation, and in the presence of ethanol or sorbate. Stress-induced nuclear localization was found to be rapid, reversible, and independent of protein synthesis. Nuclear localization of Msn2p and Msn4p was shown to be correlated inversely to cAMP levels and protein kinase A (PKA) activity. A region with significant homologies shared between Msn2p and Msn4p is sufficient to confer stress-regulated localization to a SV40-NLS-GFP fusion protein. Serine to alanine or aspartate substitutions in a conserved PKA consensus site abolished cAMP-driven nuclear export and cytoplasmic localization in unstressed cells. We propose stress and cAMP-regulated intracellular localization of Msn2p to be a key step in STRE-dependent transcription and in the general stress response.

Protein synthesis by native chemical ligation: expanded scope by using straightforward methodology

The total chemical synthesis of proteins has great potential for increasing our understanding of the molecular basis of protein function. The introduction of native chemical ligation techniques to join unprotected peptides next to a cysteine residue has greatly facilitated the synthesis of proteins of moderate size. Here, we describe a straightforward methodology that has enabled us to rapidly analyze the compatibility of the native chemical ligation strategy for X-Cys ligation sites, where X is any of the 20 naturally occurring amino acids. The simplified methodology avoids the necessity of specific amino acid thioester linkers or alkylation of C-terminal thioacid peptides. Experiments using matrix-assisted laser-desorption ionization MS analysis of combinatorial ligations of LYRAX-C-terminal thioester peptides to the peptide CRANK show that all 20 amino acids are suitable for ligation, with Val, Ile, and Pro representing less favorable choices because of slow ligation rates. To illustrate the method's utility, two 124-aa proteins were manually synthesized by using a three-step, four-piece ligation to yield a fully active human secretory phospholipase A(2) and a catalytically inactive analog. The combination of flexibility in design with general access because of simplified methodology broadens the applicability and versatility of chemical protein synthesis.

Peroxynitrite-mediated oxidation of dihydrorhodamine 123

Nitric oxide reacts with superoxide to form peroxynitrite, which may be an important mediator of free radical-induced cellular injury. Oxidation of dihydrorhodamine to fluorescent rhodamine is a marker of cellular oxidant production. We investigated the mechanisms of peroxynitrite-mediated formation of rhodamine from dihydrorhodamine. Peroxynitrite at low levels (0-1000 nM) induced a linear, concentration-dependent, oxidation of dihydrorhodamine. Hydroxyl radical scavengers mannitol and dimethylsulfoxide had minimal effect (< 10%) on rhodamine production. Peroxynitrite-mediated formation of rhodamine was not dependent on metal ion catalyzed reactions because studies were performed in metal ion-free buffer and rhodamine formation was not enhanced in the presence of Fe3+ ethylenediaminetetraacetic acid (EDTA). Thus, rhodamine formation appears to be mediated directly by peroxynitrite. Superoxide dismutase slightly enhanced rhodamine production. L-cysteine was an efficient inhibitor (KI approximately 25 microM) of dihydrorhodamine oxidation through competetive oxidation of free sulfhydryls. Urate was also an efficient inhibitor (KI approximately 2.5 microM), possibly by reduction of an intermediate dihydrorhodamine radical and recycling of dihydrorhodamine. Under anaerobic conditions, nitric oxide did not oxidize dihydrorhodamine and inhibited spontaneous oxidation of dihydrorhodamine. In the presence of oxygen, nitric oxide induces a relatively slow oxidation of dihydrorhodamine due to the formation of nitrogen dioxide. We conclude that dihydrorhodamine is a sensitive and efficient trap for peroxynitrite and may serve as a probe for peroxynitrite production.

Insulin gene enhancer binding protein Isl-1 is a member of a novel class of proteins containing both a homeo- and a Cys-His domain

The activity of the rat insulin I gene enhancer is mainly dependent on two cis-acting protein-binding domains. Here we report the isolation of a complementary DNA encoding a protein, Isl-1, that binds to one of these domains. Isl-1 contains a homeodomain with greatest similarity to those of the Caenorhabditis elegans proteins encoded by mec-3 and lin-11. In addition, Isl-1, like the lin-11 and mec-3 gene products, contains a novel Cys-His domain which is reminiscent of known metal-binding regions. Together these proteins define a novel class of proteins containing both a homeo- and a Cys His-domain. Isl-1 is preferentially expressed in cells of pancreatic endocrine origin. If the structural homologies between Isl-1 and the C. elegans gene products reflect functional similarities, a role for Isl-1 in the development of pancreatic endocrine cells could be envisaged.

Characterization of the human cysteinyl leukotriene 2 receptor

The contractile and inflammatory actions of the cysteinyl leukotrienes (CysLTs), LTC(4), LTD(4), and LTE(4), are thought to be mediated through at least two distinct but related CysLT G protein-coupled receptors. The human CysLT(1) receptor has been recently cloned and characterized. We describe here the cloning and characterization of the second cysteinyl leukotriene receptor, CysLT(2), a 346-amino acid protein with 38% amino acid identity to the CysLT(1) receptor. The recombinant human CysLT(2) receptor was expressed in Xenopus oocytes and HEK293T cells and shown to couple to elevation of intracellular calcium when activated by LTC(4), LTD(4), or LTE(4). Analyses of radiolabeled LTD(4) binding to the recombinant CysLT(2) receptor demonstrated high affinity binding and a rank order of potency for competition of LTC(4) = LTD(4) LTE(4). In contrast to the dual CysLT(1)/CysLT(2) antagonist, BAY u9773, the CysLT(1) receptor-selective antagonists MK-571, montelukast (Singulair(TM)), zafirlukast (Accolate(TM)), and pranlukast (Onon(TM)) exhibited low potency in competition for LTD(4) binding and as antagonists of CysLT(2) receptor signaling. CysLT(2) receptor mRNA was detected in lung macrophages and airway smooth muscle, cardiac Purkinje cells, adrenal medulla cells, peripheral blood leukocytes, and brain, and the receptor gene was mapped to chromosome 13q14, a region linked to atopic asthma.

Interaction of the unique N-terminal region of tyrosine kinase p56lck with cytoplasmic domains of CD4 and CD8 is mediated by cysteine motifs

p56lck, a lymphocyte-specific member of the src family of cytoplasmic protein-tyrosine kinases, is associated noncovalently with the cell surface glycoproteins CD4 and CD8, which are expressed on functionally distinct subpopulations of T cells. Using transient coexpression of p56lck with CD4 or CD8 alpha in COS-7 cells, we show that the unique N-terminal region of p56lck binds to the membrane-proximal 10 and 28 cytoplasmic residues of CD8 alpha and CD4, respectively. Two cysteine residues in each of the critical sequences in CD4, CD8 alpha, and p56lck are required for association. Our results suggest a novel role for cysteine-mediated interactions between unrelated proteins and provide a model for the association of other src-like cytoplasmic kinases with transmembrane proteins.

Multiple modes of activation of latent human fibroblast collagenase: evidence for the role of a Cys73 active-site zinc complex in latency and a "cysteine switch" mechanism for activation

Latent human fibroblast collagenase (HFC) can be activated by a variety of seemingly disparate means. In addition to the well-characterized activation by trypsin and organomercurial compounds, the enzyme can be activated to various extents by surfactants such as sodium dodecyl sulfate, by chaotropic ions such as SCN-, by disulfide compounds such as oxidized glutathione, by sulfhydryl alkylating agents such as N-ethylmaleimide, and by oxidants such as NaOCl. The underlying basis for these activations is the modification, exposure, or proteolytic release of the Cys73 residue from its habitat in the latent enzyme where it is thought to be complexed to the active-site zinc atom. This residue is not accessible for reaction with small molar excesses of dithionitrobenzoate in native, latent HFC. However, on addition of EDTA, this residue becomes fully exposed and is quantitatively labeled. All modes of activation of latent HFC are believed to involve the dissociation of Cys73 from the active-site zinc atom and its replacement by water, with the concomitant exposure of the active site. This is thought to be the primary event that precedes the well-known autolytic cleavages that are observed following the appearance of collagenase activity. The dissociation of Cys73 from the zinc atom in the latent enzyme "switches" the role of the zinc from a noncatalytic to a catalytic one. This "cysteine switch" mechanism of regulation may be applicable to the entire collagenase gene family.

Hepatic expression of mature transforming growth factor beta 1 in transgenic mice results in multiple tissue lesions

Aberrant expression of transforming growth factor beta 1 (TGF-beta 1) has been implicated in a number of disease processes, particularly those involving fibrotic and inflammatory lesions. To determine the in vivo effects of overexpression of TGF-beta 1 on the function and structure of hepatic as well as extrahepatic tissues, transgenic mice were generated containing a fusion gene (Alb/TGF-beta 1) consisting of modified porcine TGF-beta 1 cDNA under the control of the regulatory elements of the mouse albumin gene. Five transgenic lines were developed, all of which expressed the Alb/TGF-beta 1 transgene selectively in hepatocytes. The transgenic line 25 expressing the highest level of the transgene in the liver also had high (> 10-fold over control) plasma levels of TGF-beta 1. Hepatic fibrosis and apoptotic death of hepatocytes developed in all the transgenic lines but was more pronounced in line 25. The fibrotic process was characterized by deposition of collagen around individual hepatocytes and within the space of Disse in a radiating linear pattern. Several extrahepatic lesions developed in line 25, including glomerulonephritis and renal failure, arteritis and myocarditis, as well as atrophic changes in pancreas and testis. The results from this transgenic model strongly support the proposed etiological role for TGF-beta 1 in a variety of fibrotic and inflammatory disorders. The transgenic model may also provide an appropriate paradigm for testing therapeutic interventions aimed at neutralizing the detrimental effects of this important cytokine.

FIZZ1, a novel cysteine-rich secreted protein associated with pulmonary inflammation, defines a new gene family

Bronchoalveolar lavage fluid from mice with experimentally induced allergic pulmonary inflammation contains a novel 9.4 kDa cysteine-rich secreted protein, FIZZ1 (found in inflammatory zone). Murine (m) FIZZ1 is the founding member of a new gene family including two other murine genes expressed, respectively, in intestinal crypt epithelium and white adipose tissue, and two related human genes. In control mice, FIZZ1 mRNA and protein expression occur at low levels in a subset of bronchial epithelial cells and in non-neuronal cells adjacent to neurovascular bundles in the peribronchial stroma, and in the wall of the large and small bowel. During allergic pulmonary inflammation, mFIZZ1 expression markedly increases in hypertrophic, hyperplastic bronchial epithelium and appears in type II alveolar pneumocytes. In vitro, recombinant mFIZZ1 inhibits the nerve growth factor (NGF)-mediated survival of rat embryonic day 14 dorsal root ganglion (DRG) neurons and NGF-induced CGRP gene expression in adult rat DRG neurons. In vivo, FIZZ1 may modulate the function of neurons innervating the bronchial tree, thereby altering the local tissue response to allergic pulmonary inflammation.

Structure of ribonucleotide reductase protein R1

Ribonucleotide reductase is the only enzyme that catalyses de novo formation of deoxyribonucleotides and is thus a key enzyme in DNA synthesis. The radical-based reaction involves five cysteins. Two redox-active cysteines are located at adjacent antiparallel strands in a new type of ten-stranded alpha/beta-barrel, and two others at the carboxyl end in a flexible arm. The fifth cysteine, in a loop in the centre of the barrel, is positioned to initiate the radical reaction.

A COLORIMETRIC MICRO-METHOD FOR THE DETERMINATION OF GLUTATHIONE

1. A rapid colorimetric and apparently specific micromethod for the determination of total glutathione in small amounts of tissue is described. Generally, less than 30mg. of tissue is sufficient and this is homogenized in ice-cold 3% metaphosphoric acid. The product is filtered through sintered glass and neutralized or diluted before being added to a cuvette containing phosphate buffer, pH7.1, 5,5'-dithiobis-(2-nitrobenzoic acid), EDTA and glutathione reductase. Addition of NADPH(2) to the system initiates a progressive reduction of 5,5'-dithiobis-(2-nitrobenzoic acid) by catalytic amounts of GSH, and this causes a colour increase at 412mmu. The rate of this change, calculated over 5min., is proportional to the total amount of glutathione present, and consequently unknown concentrations may be determined by reference to standards. 2. A preparation (based on that of Racker, 1955) of a suitable sample of glutathione reductase from yeast is described. 3. A less specific and less sensitive determination of extracted thiol groups with 5,5'-dithiobis-(2-nitrobenzoic acid) at pH8.0, based on observations of Ellman (1959) and Jocelyn (1962), is also described. 4. Although the precise nature of the reaction is not known, evidence is put forward to support a process of cyclo-reduction. GSSG is reduced enzymically to GSH, which reacts with 5,5'-dithiobis-(2-nitrobenzoic acid) to produce a coloured ion: [Formula: see text] (E(max.) 412mmu) and a mixed disulphide. This disulphide reacts with further quantities of GSH to liberate another ion and GSSG, which then re-enters the cycle.

Upregulation of Nox4 by hypertrophic stimuli promotes apoptosis and mitochondrial dysfunction in cardiac myocytes

RATIONALE

NADPH oxidases are a major source of superoxide (O(2)(-)) in the cardiovascular system. The function of Nox4, a member of the Nox family of NADPH oxidases, in the heart is poorly understood.

OBJECTIVE

The goal of this study was to elucidate the role of Nox4 in mediating oxidative stress and growth/death in the heart.

METHODS AND RESULTS

Expression of Nox4 in the heart was increased in response to hypertrophic stimuli and aging. Neither transgenic mice with cardiac specific overexpression of Nox4 (Tg-Nox4) nor those with catalytically inactive Nox4 (Tg-Nox4-P437H) showed an obvious baseline cardiac phenotype at young ages. Tg-Nox4 gradually displayed decreased left ventricular (LV) function with enhanced O(2)(-) production in the heart, which was accompanied by increased apoptosis and fibrosis at 13 to 14 months of age. On the other hand, the level of oxidative stress was attenuated in Tg-Nox4-P437H. Although the size of cardiac myocytes was significantly greater in Tg-Nox4 than in nontransgenic, the LV weight/tibial length was not significantly altered in Tg-Nox4 mice. Overexpression of Nox4 in cultured cardiac myocytes induced apoptotic cell death but not hypertrophy. Nox4 is primarily localized in mitochondria and upregulation of Nox4 enhanced both rotenone- and diphenyleneiodonium-sensitive O(2)(-) production in mitochondria. Cysteine residues in mitochondrial proteins, including aconitase and NADH dehydrogenases, were oxidized and their activities decreased in Tg-Nox4.

CONCLUSIONS

Upregulation of Nox4 by hypertrophic stimuli and aging induces oxidative stress, apoptosis and LV dysfunction, in part because of mitochondrial insufficiency caused by increased O(2)(-) production and consequent cysteine oxidation in mitochondrial proteins.

Novel cysteine-rich motif and homeodomain in the product of the Caenorhabditis elegans cell lineage gene lin-11

The gene lin-11 is required for the asymmetric division of a vulval precursor cell type in the nematode Caenorhabditis elegans. Putative lin-11 complementary DNAs were sequenced and found to encode a protein that contains both a homeodomain and two tandem copies of a novel cysteine-rich motif: C-X2-C-X17-19-H-X2-C-X2-C-X2-C-X7-11-(C)-X8-C. Two tandem copies of this motif are also present amino-terminal to the homeodomains in the proteins encoded by the genes mec-3, which is required for C. elegans touch neuron differentiation, and isl-1, which encodes a rat insulin I gene enhancer-binding protein. The arrangement of cysteine residues in this motif, referred to as LIM (for lin-11 isl-1 mec-3), suggests that this region is a metal-binding domain. The presence in these three proteins of both a potential metal-binding domain and a homeodomain distinguishes them from previously characterized proteins.