S100B proteins that lack one or both cysteine residues can induce inflammatory responses in astrocytes and microglia

The astrocytic protein S100B stimulates neurite outgrowth and neuronal survival during CNS development. S100B can also stimulate glial activation, leading to induction of pro-inflammatory molecules like interleukin-1 beta (IL-1 beta) and inducible nitric oxide synthase (iNOS). Although it is known that S100B's neurotrophic activity requires a disulfide-linked dimeric form of the protein, the structural features of S100B that are important for glial activation have not been defined. As an initial step towards understanding the structural features of S100B required for its action on glia and to determine if these features are different from those required for its action on neurons, we tested two mutants of S100B for their ability to activate glia. The C68VC84S mutant lacks S100B's two cysteine residues (cys68, cys84) and lacks neurotrophic activity (Winningham-Major et al., 1989, J. Cell Biol. 109 3063-3071), and the truncation mutant S100B83stop lacks the C-terminal nine residues (including cys84) that have been shown to be important for some S100B:target protein interactions. We report here that both C68VC84S and S100B83stop stimulate glial activation, as determined by induction of iNOS and IL-1 beta in rat primary astrocyte and microglial cultures. C68VC84S showed activation profiles similar to those of wild-type S100B, demonstrating that a disulfide-linked dimer is not required for glial activation. S100B83stop also stimulated both iNOS and IL-1 beta, although S100B83stop was significantly less effective than wild-type S100B in inducing iNOS. These results indicate that the C-terminal region of S100B is not required for glial activation; however, its presence may influence the degree of activation by the protein. Altogether, these studies demonstrate that the structural features required for S100B's neurotrophic activity are distinct from those affecting its glial activation activity.     

S100 is present in developing chicken neurons and Schwann cells and promotes motor neuron survival in vivo.

We used polyclonal antisera recognizing S100, a small acidic protein highly enriched in nervous tissue, to stain sections of embryonic chicken lumbosacral spinal cord and hindlimb. S100 immunoreactivity was detected in developing sensory neurons of the dorsal root ganglia (DRG) and motor neurons of the ventral spinal cord as early as embryonic day (E) 5, and staining persisted through hatching. In contrast, expression of S100 first became apparent in Schwann cells at E13, just before myelination, and was not detected in developing skin or muscle. Since S100 beta was present in motor and sensory neurons and is known to promote neuronal survival and neurite extension in vitro (Winningham-Major, Staecker, Barger, Coats, and Van Eldik, 1989), we tested the ability of S100 to promote neuron survival in an in ovo survival assay. Addition of S100 to chick embryos in ovo during the period of naturally occurring motor neuron cell death resulted in a significant increase in motor neuron survival, but had no effect on the in vivo survival of sensory neurons in the DRG. The findings that S100 is present in spinal motor neurons and that the addition of S100 enhances the survival of these cells in vivo are consistent with the possibility that S100 may act as a naturally occurring neuron survival factor during development.     

Synthesis and expression of a gene coding for the calcium-modulated protein S100 beta and designed for cassette-based, site-directed mutagenesis

As an initial step in studies aimed at addressing the question of what common and unique features of the S100 family of proteins are related to their specific functions and localizations, a gene coding for one of the S100 proteins, S100 beta, has been prepared by ligation of 12 overlapping, synthetic oligonucleotides. Automated DNA sequence analysis demonstrated that the final construct has the expected structure. The gene was inserted into a plasmid vector that contains a tac promoter and ampicillin-resistance gene, thus allowing both amplification and direct expression cloning in Escherichia coli. The gene was designed to allow rapid, efficient changes of single or multiple amino acids by using cassette-based mutagenesis while the gene is resident in the vector. The expressed protein (VUSB-1) is indistinguishable from bovine brain S100 beta in terms of electrophoretic mobility, reactivity with antibodies to S100 beta, amino acid composition, and partial amino acid sequence analysis. Preparations of expressed protein are also functionally similar to bovine brain S100 beta as determined by aldolase activator activity and neurite extension factor activity, supporting the concept that these activities are a property of the S100 beta polypeptide.     

A cAMP-activated pathway, including PKA and PI3K, regulates neuronal differentiation

Neuronal differentiation is a complex process in which many different signalling pathways may be involved. An increase in the intracellular levels of cyclic AMP (cAMP) has been shown to induce neuronal differentiation and also to cooperate with NGF to induce PC12 neurite outgrowth in a Ras-dependent manner. However, the neuritogenic activities associated with cAMP are still not well understood. The purpose of this study was to investigate the potential neuritogenic activities mediated by cAMP. For this purpose, we used the human neuroblastoma cell line SH-SY5Y. These neuroblastoma cells respond to cAMP by forming neurite-like extensions. We tried to identify some essential pathways involved in the cAMP-induced neurite elongation of these cells. Our results indicated that PKA is transiently activated in this elongation model. When we blocked PKA activity, elongation did not take place. Similarly, PI3K also plays an essential role because when we blocked this kinase activity, there was no neurite elongation. Indeed, over-expression of the p110-catalytic subunit or an activating form of the p85-regulatory subunit (p65) is able to induce some degree of neurite extension. Moreover, our results showed that when elongation is initiated, PI3K is still essential for maintenance of the neuronal morphology, whereas PKA or MAPK (ERKs or p38) activation does not appear to be necessary during this process.     

Adapter protein SH2-B beta undergoes nucleocytoplasmic shuttling: implications for nerve growth factor induction of neuronal differentiation

The adapter protein SH2-B has been shown to bind to activated nerve growth factor (NGF) receptor TrkA and has been implicated in NGF-induced neuronal differentiation and the survival of sympathetic neurons. However, the mechanism by which SH2-B enhances and maintains neurite outgrowth is unclear. We examined the ability of truncation mutants to regulate neuronal differentiation and observed that certain truncation mutants localized in the nucleus rather than in the cytoplasm or at the plasma membrane as reported for wild-type SH2-B beta. Addition of the nuclear export inhibitor leptomycin B caused both overexpressed wild-type and endogenous SH2-B beta to accumulate in the nucleus of both PC12 cells and COS-7 cells as did deletion of a putative nuclear export sequence (amino acids 224 to 233) or mutation of two critical lysines in that sequence. Deleting or mutating the nuclear export signal caused SH2-B beta to lose its ability to enhance NGF-induced differentiation of PC12 cells. Neither the NGF-induced phosphorylation of ERKs 1 and 2 nor their subcellular distribution was altered in PC12 cells stably expressing the nuclear export-defective SH2-B beta(L231A, L233A). These data provide strong evidence that SH2-B beta shuttles constitutively between the nucleus and cytoplasm. However, SH2-B beta needs continuous access to the cytoplasm and/or plasma membrane to participate in NGF-induced neurite outgrowth. These data also suggest that the stimulatory effect of SH2-B beta on NGF-induced neurite outgrowth of PC12 cells is either downstream of ERKs or via some other pathway yet to be identified.     

Molecular mechanisms of Ca(2+) signaling in neurons induced by the S100A4 protein

The S100A4 protein belongs to the S100 family of vertebrate-specific proteins possessing both intra- and extracellular functions. In the nervous system, high levels of S100A4 expression are observed at sites of neurogenesis and lesions, suggesting a role of the protein in neuronal plasticity. Extracellular oligomeric S100A4 is a potent promoter of neurite outgrowth and survival from cultured primary neurons; however, the molecular mechanism of this effect has not been established. Here we demonstrate that oligomeric S100A4 increases the intracellular calcium concentration in primary neurons. We present evidence that both S100A4-induced Ca(2+) signaling and neurite extension require activation of a cascade including a heterotrimeric G protein(s), phosphoinositide-specific phospholipase C, and diacylglycerol-lipase, resulting in Ca(2+) entry via nonselective cation channels and via T- and L-type voltage-gated Ca(2+) channels. We demonstrate that S100A4-induced neurite outgrowth is not mediated by the receptor for advanced glycation end products, a known target for other extracellular S100 proteins. However, S100A4-induced signaling depends on interactions with heparan sulfate proteoglycans at the cell surface. Thus, glycosaminoglycans may act as coreceptors of S100 proteins in neurons. This may provide a mechanism by which S100 proteins could locally regulate neuronal plasticity in connection with brain lesions and neurological disorders.     

Calmodulin binding and Cdk5 phosphorylation of p35 regulate its effect on microtubules

In the nervous system, Cdk5 and its neuronal activator p35 are involved in the control of various activities, including neuronal differentiation and migration. Recently, we have reported that p35 is a microtubule-associated protein that regulates microtubule dynamics ( Hou, Z., Li, Q., He, L., Lim, H. Y., Fu, X., Cheung, N. S., Qi, D. X., and Qi, R. Z. (2007) J. Biol. Chem. 282, 18666-18670 ). Here we present two regulatory modes of p35 function as a microtubule-associated protein. First, p35 is Ca(2+)-dependent calmodulin (CaM)-binding protein. The CaM- and microtubule binding domains are localized to overlapping regions at the N terminus of p35. Within the CaM-binding region, Ala substitution for Trp-52 abolishes the CaM-binding activity, corroborating specific CaM-binding of p35. Furthermore, CaM blocks p35 association with microtubules in a Ca(2+)-specific manner, suggesting that p35 may be involved in the Ca(2+)/CaM-mediated inhibition of microtubule assembly. Second, p35 phosphorylation by Cdk5 interferes with the microtubule-binding and polymerizing activities of p35. Using a mutational approach, we found that only phosphorylation at Thr-138, one of the two residues primarily phosphorylated in vivo, inhibits the polymerizing activity. In PC12 cells, expression of p35 promotes nerve growth factor-induced neurite outgrowth under a Cdk5 inhibitory condition. Such p35 activity is impaired by the phosphomimetic mutation of Thr-138. These data suggest that Thr-138 phosphorylation plays a critical role in the control of the p35 functions in microtubule assembly and neurite outgrowth.     

Phosphatidylcholine biosynthesis via CTP:phosphocholine cytidylyltransferase 2 facilitates neurite outgrowth and branching

Hallmarks of neuronal differentiation are neurite sprouting, extension, and branching. We previously showed that increased expression of CTP:phosphocholine cytidylyltransferase beta2 (CTbeta2), an isoform of a key phosphatidylcholine (PC) biosynthetic enzyme, accompanies neurite outgrowth (Carter, J. M., Waite, K. A., Campenot, R. B., Vance, J. E., and Vance, D. E. (2003) J. Biol. Chem. 278, 44988-44994). CTbeta2 mRNA is highly expressed in the brain. We show that CTbeta2 is abundant in axons of rat sympathetic neurons and retinal ganglion cells. We used RNA silencing to decrease CTbeta2 expression in PC12 cells differentiated by nerve growth factor. In CTbeta2-silenced cells, numbers of primary and secondary neurites were markedly reduced, suggesting that CTbeta2 facilitates neurite outgrowth and branching. However, the length of individual neurites was significantly increased, and the total amount of neuronal membrane was unchanged. Neurite branching of PC12 cells is known to be inhibited by activation of Akt and promoted by the Akt inhibitor LY294002. Our experiments showed that LY294002 increases neurite sprouting and branching in control PC12 cells but not in CTbeta2-deficient cells. CTbeta2 was not phosphorylated in vitro by Akt. However, inhibition of Cdk5 by roscovitine blocked CTbeta2 phosphorylation and reduced neurite outgrowth and branching. These results highlight the importance of CTbeta2 in neurons for promoting neurite outgrowth and branching and represent the first identification of a lipid biosynthetic enzyme that facilitates these functions.     

S100β Increases Levels of β-Amyloid Precursor Protein and Its Encoding mRNA in Rat Neuronal Cultures

Abstract: S100β has been implicated in the formation of dystrophic neurites, overexpressing β-amyloid precursor protein (βAPP), in the β-amyloid plaques of Alzheimer's disease. We assessed the effects of S100β on cell viability of, neurite outgrowth from, and βAPP expression by neurons in primary cultures from fetal rat cortex. S100β (1–10 ng/ml) enhanced neuronal viability (as assessed by increased mitochondrial activity and decreased lactic acid dehydrogenase release) and promoted neurite outgrowth. Higher levels of S100β (100 ng/ml, but not 1 µg/ml) produced qualitatively similar, but less marked, effects. S100β also induced increased neuronal expression of the microtubule-associated protein MAP2, an effect that is consistent with trophic effects of S100β on neurite outgrowth. S100β (10 and 100 ng/ml) induced graded increases in neuronal expression of βAPP and of βAPP mRNA. These results support our previous suggestion that excessive expression of S100β by activated, plaque-associated astrocytes in Alzheimer's disease contributes to the appearance of dystrophic neurites overexpressing βAPP in diffuse amyloid deposits, and thus to the conversion of these deposits into the diagnostic neuritic β-amyloid plaques.     

Constitutively expressed catalytic proteasomal subunits are up-regulated during neuronal differentiation and required for axon initiation, elongation and maintenance

Inhibition of the proteasome by lactacystin, a specific blocker of the catalytic beta-subunits, results in transient neurite outgrowth by neuronal cell lines. Vice versa, as demonstrated in this study, treatment of pheochromocytoma (PC12) cells with nerve growth factor (NGF) or other differentiating agents reduces proteasomal activity. This is accompanied by an increase in mRNA and protein levels of the catalytically active subunits beta1, beta2 and beta5, but not of their inducible counterparts, indicating changes in subunit composition of the proteasome during neuronal differentiation. In contrast to neuronal cell lines, however, pre-treatment of primary neurons with proteasome inhibitors completely prevents axon formation, and lower concentrations of lactacystin (0.5-5 microm) significantly reduce axonal elongation and branching in vitro. Furthermore, established axonal networks degenerate rapidly and long-term survival of peripheral neurons is impaired in the presence of proteasome inhibitors. Axonal pathology is reminiscent of the morphological changes observed in neurodegenerative disorders and supports a crucial role of the constitutive catalytic subunits in axon initiation, maintenance and regeneration.     

Structural and functional insights into RAGE activation by multimeric S100B

Nervous system development and plasticity require regulation of cell proliferation, survival, neurite outgrowth and synapse formation by specific extracellular factors. The EF-hand protein S100B is highly expressed in human brain. In the extracellular space, it promotes neurite extension and neuron survival via the receptor RAGE (receptor for advanced glycation end products). The X-ray structure of human Ca(2+)-loaded S100B was determined at 1.9 A resolution. The structure revealed an octameric architecture of four homodimeric units arranged as two tetramers in a tight array. The presence of multimeric forms in human brain extracts was confirmed by size-exclusion experiments. Recombinant tetrameric, hexameric and octameric S100B were purified from Escherichia coli and characterised. Binding studies show that tetrameric S100B binds RAGE with higher affinity than dimeric S100B. Analytical ultracentrifugation studies imply that S100B tetramer binds two RAGE molecules via the V-domain. In line with these experiments, S100B tetramer caused stronger activation of cell growth than S100B dimer and promoted cell survival. The structural and the binding data suggest that tetrameric S100B triggers RAGE activation by receptor dimerisation.     

Trophic effect of beta-amyloid precursor protein on cerebral cortical neurons in culture

We investigated the effect of human beta-amyloid precursor protein (APP) on rat primary cerebral cortical neurons cultured in a serum-free medium. Two secretory APP species (APP667 and APP592) with and without the protease inhibitor domain were produced by COS-1 cells transfected with APP cDNAs, which encode the N-terminal portions of APP770 and APP695. Both highly purified APP species, when added to the medium, enhanced neuronal survival and neurite extension in a dose-dependent manner with a maximum effect at approximately 100 nM. These results suggest that secreted forms of APP have trophic activity for cerebral cortical neurons.     

Beta 3: a new member of nicotinic acetylcholine receptor gene family is expressed in brain

Screening of a rat brain cDNA library with a radiolabeled probe made from an alpha 3 cDNA (Boulter, J., Evans, K., Goldman, D., Martin, G., Treco, D., Heinemanns, S., and Patrick, J. (1986) Nature 319, 368-374) resulted in the isolation of a clone whose sequence encodes a protein, beta 3, which is homologous (40-55% amino acid sequence identity) to previously described neuronal nicotinic acetylcholine receptor subunits. The encoded protein has structural features found in other nicotinic acetylcholine receptor (nAChR) subunits. Two cysteine residues that correspond to cysteins 128 and 142 of the Torpedo nAChR alpha subunit are present in beta 3. Absent from beta 3 are 2 adjacent cysteine residues that correspond to cysteines 192 and 193 of the Torpedo subunit. In situ hybridization histochemistry, performed using probes derived from beta 3 cDNAs, demonstrated that the beta 3 gene is expressed in the brain. Thus, beta 3 is the fifth member of the nAChR gene family that is expressed in the brain. The pattern of beta 3 gene expression partially overlaps with that of the neuronal nAChR subunit genes alpha 3, alpha 4, or beta 2. These results lead us to propose that the beta 3 gene encodes a neuronal nAChR subunit.     

Effects of astroglia on the development of cultured neurons from embryonic rat cerebral cortex.

1. Previous studies from our laboratory demonstrated that subcultured astroglia enhance neurite outgrowth and survival of cultured neurons from embryonic rat cerebral cortex, but suppress proliferation of neuroblasts. 2. In the present study, the mechanisms of these three effects were further investigated. 3. Dissociated neurons were seeded on poly-L-lysine-coated coverslips which were plated on subcultured astroglia, and the survival, proliferation and neurite outgrowth of the neurons were investigated. Under these conditions, survival and antimitotic effects were also observed, while neurite extension was not stimulated. 4. The results clearly indicate that neuronal survival and proliferation are regulated by soluble factors produced by astroglia. 5. We also postulated that the neurite-promoting effect of astroglia is mediated by cell-cell contact. 6. This idea was confirmed by the finding that neurite extension was enhanced when the neurons were cultured directly on heat-treated astroglia. 7. The neurite-promoting effect was found to be specific to astroglia. 8. We preliminarily characterized the astroglial surface neurite-promoting factors (ASNPFs). 9. The relationship of laminin to ASNPFs was examined by using antibody to laminin. Laminin antibody did not inhibit the ASNPF activity. 10. The effect of digestion of heat-treated astroglia with enzymes (sialidase and endo-beta-galactosidase) on the ASNPF activity was also examined. 11. These enzyme treatments did not inhibit the ASNPF activity. 12. These results suggest that enhancement of the neurite-promoting activity is not associated with the sugar moiety of ASNPFs.     

Neurite outgrowth in response to transfected N-CAM and N-cadherin reveals fundamental differences in neuronal responsiveness to CAMs

Different neuronal populations were used to compare the neurite outgrowth-promoting activities of N-CAM and N-cadherin expressed via gene transfer on the surface of nonneuronal cells. In contrast to a previously reported developmental loss of retinal ganglion cell responsiveness to N-CAM, these cells exhibited an increased and maintained responsiveness to N-cadherin over the same developmental period (E6-E11). N-CAM and N-cadherin responses could be specifically inhibited by their own antibodies, but not by antisera to the beta 1 integrin family or the L1/G4 glycoprotein. Cerebellar neurons showed qualitative differences in the nature of the dose-response curves for transfected N-CAM expression (highly cooperative) versus N-cadherin expression (linear). In addition "subthreshold" levels of N-CAM expression, which do not normally support neurite outgrowth, did so when coexpressed with functional levels of N-cadherin. These studies show fundamental differences in neuronal responsiveness to cell adhesion molecules and suggest a more dynamic regulation for N-CAM-dependent neurite outgrowth than for N-cadherin-dependent outgrowth.     

Developmental changes in the protective effect of exogenous brain-derived neurotrophic factor and neurotrophin-3 against ototoxic drugs in cultured rat vestibular ganglion neurons

We examine developmental changes in the responsiveness of rat vestibular ganglion neurons (VGNs) to two neurotrophic factors (NTFs), brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3) and investigate the protective effects of these NTFs against ototoxic drugs during postnatal development in dissociated cultures. VGNs were obtained from rats on postnatal days (P) 1, 3, 7 and 14. BDNF facilitated neuronal survival as well as neurite sprouting of VGNs obtained from younger rats (P1 and P3), whereas these effects were not observed in older rats (P7 and P14). BDNF was also effective in facilitating neurite extension in VGNs at each of the postnatal ages. NT-3 also facilitated neuronal survival and neurite extension of VGNs from younger rats but these effects were significantly smaller than those of BDNF (p?<?0.05). The protective effects of BDNF and NT-3 against ototoxic drugs, gentamicin and cisplatin, were also age-dependent: they were effective for neuronal survival, neurite sprouting and neurite extension in VGNs from younger rats, whereas these effects tended to disappear in VGNs from older rats. Analysis of the changes in the expression of the receptors of NTFs revealed that expression of TrkB and TrkC proteins and their mRNA did not change during the developmental period, whereas expression of p75^NTR protein was down-regulated together with that of p75^NTR mRNA during the developmental period. Developmental changes in the responsiveness to exogenous NTFs in VGNs, which is not caused by the changes of their receptors but probably caused by changes in the intracellular signaling pathways, should be taken into consideration in the prevention of neuronal degeneration caused by ototoxic drugs.     

S100 is present in developing chicken neurons and schwann cell and promotes motor neuron survival in vivo

We used polyclonal antisera recognizing S100, a small acidic protein highly enriched in nervous tissue, to stain sections of embryonic chicken lumbosacral spinal cord and hindlimb. S100 immunoreactivity was detected in developing sensory neurons of the dorsal root ganglia (DRG) and motor neurons of the ventral spinal cord as early as embryonic day (E) 5, and staining persisted through hatching. In contrast, expression of S100 first became apparent in Schwann cells at E13, just before myelination, and was not detected in developing skin or muscle. Since S100β was present in motor and sensory neurons and is known to promote neuronal survival and neurite extension in vitro (Winningham-Major, Staecker, Barger, Coats, and Van Eldik, 1989), we tested the ability of S100 to promote neuron survival in an in ovo survival assay. Addition of S100 to chick embryos in ovo during the period of naturally occurring motor neuron cell death resulted in a significant increase in motor neuron survival, but had no effect on the in vivo survival of sensory neurons in the DRG. The findings that S100 is present in spinal motor neurons and that the addition of S100 enhances the survival of these cells in vivo are consistent with the possibility that S100 may act as a naturally occurring neuron survival factor during development. © 1992 John Wiley & Sons, Inc.     

Recognition of the A chain carboxy-terminal heparin binding region of fibronectin involves multiple sites: two contiguous sequences act independently to promote neural cell adhesion

Cellular interactions with fibronectin-treated substrata have a complex molecular basis involving multiple domains. A carboxy-terminal cell and heparin binding region of fibronectin (FN) is particularly interesting because it is a strong promoter of neurite outgrowth (Rogers, S.L., J.B. McCarthy, S.L. Palm, L.T. Furcht, and P.C. Letourneau, 1985. J. Neurosci. 5:369-378) and cell attachment (McCarthy, J.B., S.T. Hagen, and L.T. Furcht. 1986. J. Cell Biol. 102:179-188). To further understand the molecular mechanisms of neuronal interactions with this region of FN, we screened two peptides from the 33-kD heparin binding fragment of the FN A chain, FN-C/H II (KNNQKSEPLIGRKKT) and CS1 (Humphries, M.J., A. Komoriya, S.K. Akiyama, K. Olden, and K.M. Yamada. 1987. J. Biol. Chem. 262:6886-6892), for their ability to promote B104 neuroblastoma cell-substratum adhesion and neurite outgrowth. Both FN- C/H II and CS1 promoted B104 cell attachment in a concentration- dependent and saturable manner, with attachment to FN-C/H II exceeding attachment to CS1. In solution, both exogenous FN-C/H II or CS1 partially inhibited cell adhesion to the 33-kD fragment. Similar results were obtained with anti-FN-C/H II antibodies. In contrast, soluble GRGDSP did not affect B104 cell adhesion to FN-C/H II. These results indicate that both FN-C/H II and CS1 represent distinct, RGD- independent, cell adhesion-promoting sites active within the 33-kD fragment, and further define FN-C/H II as a novel neural recognition sequence in FN. B104 adhesion to FN-C/H II and CS1 differs in sensitivity to heparin, yet each peptide inhibited adhesion to the other peptide, suggesting cell adhesion is somehow related at the cellular level. Within the A chain 33-kD fragment, FN-C/H II and CS1 are contiguous, and might represent components of a larger domain with greater neurite-promoting activity since only the 33-kD fragment, and neither individual peptide, was effective at promoting B104 neurite outgrowth. These data further support the hypothesis that cell responses to FN are mediated by multiple sites involving both heparin- sensitive and -insensitive mechanisms.     

Reactive sulfhydryl groups of alpha 39, a guanine nucleotide-binding protein from brain. Location and function

The guanine nucleotide-binding proteins which mediate hormonal inhibition of adenylate cyclase as well as hormonal regulation of other membrane functions are alpha, beta, and gamma heterotrimers which are structurally homologous to each other. In brain, the predominant guanine nucleotide-binding component is a 39-kDa protein whose physiological role is as yet unknown. We have used N-ethylmaleimide to define functionally important sulfhydryl groups on alpha 39. Three cysteine residues in the molecule are reactive in unliganded alpha 39. Alkylation of two of these is reduced when guanosine 5'-(3'-O-thio)triphosphate (GTP gamma S) is bound. We have isolated and sequenced tryptic peptides containing the three reactive cysteines. The octapeptide containing the GTP gamma S-insensitive cysteine is at a position equivalent to amino acids 106-113 of the transducin alpha subunit (Lochrie, M. A., Hurley, J. B., and Simon, M. I. (1985) Science 228, 96-99). However, the equivalent peptide in transducin does not contain a cysteine residue. Alkylation of this cysteine blocks ADP-ribosylation of cysteine 351 by pertussis toxin. However, alkylation does not prevent association of alpha with the beta X gamma subunits nor does it inhibit GTPase activity. The two GTP gamma S-sensitive cysteines are at positions equivalent to cysteines 139 and 286 of the transducin alpha subunit. Alkylation of these residues inhibits GTPase activity. Neither of these GTP gamma S-sensitive cysteines are in those regions of alpha 39 which are highly homologous to the GTP-binding site of elongation factor Tu (Jurnak, F. (1985) Science 230, 32-36). However, both are present in the brain 41-kDa guanine nucleotide-binding protein and in the two transducins. The conservation of these cysteine residues suggests that they are important for the function of the subunits.