Measurement of the binding of tyrosyl phosphopeptides to SH2 domains: a reappraisal

Src homology 2 (SH2) domain-mediated interactions with phosphotyrosine residues are critical in many intracellular signal transduction pathways. Attempts to understand the determinants of specificity and selectivity of these interactions have prompted many binding studies that have used several techniques. Some discrepancies, in both the absolute and relative values of the dissociation constants for particular interactions, are apparent. To establish the correct dissociation constants and to understand the origin of these differences, we have analyzed three previously determined interactions using the techniques of surface plasmon resonance and isothermal titration calorimetry. We find that the binding of SH2 domains to phosphopeptides is weaker than generally presumed. A phosphopeptide based on the hamster polyoma middle tumor antigen interacts with the SH2 domain from Src with an equilibrium dissociation constant (Kd) of 600 nM; a phosphopeptide based on one binding site from the platelet-derived growth factor receptor binds to the N-terminal SH2 domain of the 1-phosphatidylinositol 3-kinase p85 subunit with a Kd of 300 nM; and a phosphopeptide based on the C terminus of Lck binds to the SH2 domain of Lck with a Kd of 4 microM. In addition, we demonstrate that avidity effects that result from the dimerization of glutathione S-transferase fusion proteins with SH2 domains could be responsible for overestimates of affinities for these interactions previously studied by surface plasmon resonance.

Identification of Src, Fyn, Lyn, PI3K and Abl SH3 domain ligands using phage display libraries

Many proteins involved in intracellular signal transduction contain a small, 50-60 amino acid domain, termed the Src homology 3 (SH3) domain. This domain appears to mediate critical protein-protein interactions that are involved in responses to extracellular signals. Previous studies have shown that the SH3 domains from several proteins recognize short, contiguous amino acid sequences that are rich in proline residues. While all SH3 recognition sequences identified to date share a conserved P-X-X-P motif, the sequence recognition specificity of individual SH3 domains is poorly understood. We have employed a novel modification of phage display involving biased libraries to identify peptide ligands of the Src, Fyn, Lyn, PI3K and Abl SH3 domains. With biased libraries, we probed SH3 recognition over a 12 amino acid window. The Src SH3 domain prefers the sequence XXXRPLPPLPXP, Fyn prefers XXXRPLPP(I/L)PXX, Lyn prefers RXXRPLPPLPXP, PI3K prefers RXXRPLPPLPP while the Abl SH3 domain selects phage containing the sequence PPPYPPPP(I/V)PXX. We have also analysed the binding properties of Abl and Src SH3 ligands. We find that although the phage-displayed Abl and Src SH3 ligands are proline rich, they are distinct. In surface plasmon resonance binding assays, these SH3 domains displayed highly selective binding to their cognate ligands when the sequences were displayed on the surface of the phage or as synthetic peptides. The selection of these high affinity SH3 peptide ligands provides valuable information on the recognition motifs of SH3 domains, serve as new tools to interfere with the cellular functions of SH3 domain-mediated processes and form the basis for the design of SH3-specific inhibitors of disease pathways.

Tandem SH2 domains confer high specificity in tyrosine kinase signaling

SH2 domain proteins transmit intracellular signals initiated by activated tyrosine kinase-linked receptors. Recent three-dimensional structures suggest mechanisms by which tandem SH2 domains might confer higher specificity than individual SH2 domains. To test this, binding studies were conducted with tandem domains from the five signaling enzymes: phosphatidylinositol 3-kinase p85, ZAP-70, Syk, SHP-2, and phospholipase C-gamma1. Bisphosphorylated TAMs (tyrosine-based activation motifs) were derived from biologically relevant sites in platelet-derived growth factor, T cell, B cell, and high affinity IgE receptors and the receptor substrates IRS-1 (insulin receptor substrate-1) and SHPS-1/SIRP. Each tandem SH2 domain binds a distinct TAM corresponding to its appropriate biological partner with highest affinity (0.5-3.0 nM). Alternative TAMs bind the tandem SH2 domains with 1,000- to >10,000-fold lower affinity than biologically relevant TAMs. This level of specificity is significantly greater than the approximately 20-50-fold typically seen for individual SH2 domains. We conclude that high biological specificity is conferred by the simultaneous interaction of two SH2 domains in a signaling enzyme with bisphosphorylated TAMs in activated receptors and substrates.

Phage display selection of ligand residues important for Src homology 3 domain binding specificity

The Src homology 3 (SH3) domain is a 50-aa modular unit present in many cellular proteins involved in intracellular signal transduction. It functions to direct protein-protein interactions through the recognition of proline-rich motifs on associated proteins. SH3 domains are important regulatory elements that have been demonstrated to specify distinct regulatory pathways important for cell growth, migration, differentiation, and responses to the external milieu. By the use of synthetic peptides, ligands have been shown to consist of a minimum core sequence and to bind to SH3 domains in one of two pseudosymmetrical orientations, class I and class II. The class I sites have the consensus sequence ZP(L/P)PP psi P whereas the class II consensus is PP psi PPZ (where psi is a hydrophobic residue and Z is a SH3 domain-specific residue). We previously showed by M13 phage display that the Src, Fyn, Lyn, and phosphatidylinositol 3-kinase (PI3K) SH3 domains preferred the same class I-type core binding sequence, RPLPP psi P. These results failed to explain the specificity for cellular proteins displayed by SH3 domains in cells. In the current study, class I and class II core ligand sequences were displayed on the surface of bacteriophage M13 with five random residues placed either N- or C-terminal of core ligand residues. These libraries were screened for binding to the Src, Fyn, Lyn, Yes, and PI3K SH3 domains. By this approach, additional ligand residue preferences were identified that can increase the affinity of SH3 peptide ligands at least 20-fold compared with core peptides. The amino acids selected in the flanking sequences were similar for Src, Fyn, and Yes SH3 domains; however, Lyn and PI3K SH3 domains showed distinct binding specificities. These results indicate that residues that flank the core binding sequences shared by many SH3 domains are important determinants of SH3 binding affinity and selectivity.

SH3 domains and drug design: ligands, structure, and biological function

The ligand binding preferences, structural features, and biological function of SH3 (Src homology 3) domains are discussed. SH3 domains bind "core" Pro-rich peptide ligands (7-9 amino acids in length) in a polyproline II helical conformation in a highly conserved aromatic rich patch on the protein surface (approximately 390 A2). The ligands can interact with the protein in one of two orientations, depending on the position (N- vs C-terminal) of ligand residues binding to the SH3 selectivity pocket. Core SH3 ligands are characterized by relatively weak interactions (KD = 5-100 microM) that show little binding selectivity within SH3 families. Higher affinity, more selective contiguous ligands require additional flanking residues that bind to less conserved portions of the SH3 surface, with corresponding increase in ligand size and complexity. In contrast to peptide ligands, protein ligands of SH3 domains can exploit multiple discontiguous interactions to enhance affinity and selectivity. A protein-SH3 interaction that utilizes unique interactions may permit the design of small high affinity SH3 ligands. At present, the extended nature of the binding site and homologous nature of the core binding region among SH3 domains present key challenges for structure-based drug design.

Discovery of potent and selective SH2 inhibitors of the tyrosine kinase ZAP-70

A series of 1,2,4-oxadiazole analogues has been shown to be potent and selective SH2 inhibitors of the tyrosine kinase ZAP-70, a potential therapeutic target for immune suppression. These compounds typically are 200-400-fold more potent than the native, monophosphorylated tetrapeptide sequences. When compared with the high-affinity zeta-1-ITAM peptide (Ac-NQL-pYNELNLGRREE-pYDVLD-NH(2), wherein pY refers to phosphotyrosine) some of the best 1,2, 4-oxadiazole analogues are approximately 1 order of magnitude less active. This series of compounds displays an unprecedented level of selectivity over the closely related tyrosine kinase Syk, as well as other SH2-containing proteins such as Src and Grb2. Gel shift studies using a protein construct consisting only of C-terminal ZAP-70 SH2 demonstrate that these compounds can effectively engage this particular SH2 domain.

Solution structure of the C-terminal SH2 domain of the human tyrosine kinase Syk complexed with a phosphotyrosine pentapeptide

BACKGROUND

Recruitment of the intracellular tyrosine kinase Syk to activated immune-response receptors is a critical early step in intracellular signaling. In mast cells, Syk specifically associates with doubly phosphorylated immunoreceptor tyrosine-based activation motifs (ITAMs) that are found within the IgE receptor. The mechanism by which Syk recognizes these motifs is not fully understood. Both Syk SH2 (Src homology 2) domains are required for high-affinity binding to these motifs, but the C-terminal SH2 domain (Syk-C) can function independently and can bind, in isolation, to the tyrosine-phosphorylated IgE receptor in vitro. In order to improve understanding of the cellular function of Syk, we have determined the solution structure of Syk-C complexed with a phosphotyrosine peptide derived from the gamma subunit of the IgE receptor.

RESULTS

The Syk-C:peptide structure is compared with liganded structures of both the SH2 domain of Src and the C-terminal SH2 domain of ZAP-70 (the 70 kDa zeta-associated protein). The topologies of these domains are similar, although significant differences occur in the loop regions. In the Syk-C structure, the phosphotyrosine and leucine residues of the peptide ligand interact with pockets on the protein, and the intervening residues are extended.

CONCLUSIONS

Syk-C resembles other SH2 domains in its peptide-binding interactions and overall topology, a result that is consistent with its ability to function as an independent SH2 domain in vitro. This result suggests that Syk-C plays a unique role in the intact Syk protein. The determinants of the binding affinity and selectivity of Syk-C may reside in the least-conserved structural elements that comprise the phosphotyrosine- and leucine-binding sites. These structural features can be exploited for the design of Syk-selective SH2 antagonists for the treatment of allergic disorders and asthma.

Biochemical characterization of the Oct-2 POU domain with implications for bipartite DNA recognition

B-cell specific regulation of immunoglobulin gene expression provides a model for the interaction of promoter and enhancer elements with eukaryotic sequence-specific DNA binding proteins. A critical element of this system, the octamer site (5'-ATGCAAAT-3'), is recognized by the B-cell transcription factor Oct-2. Octamer recognition is mediated by the POU domain, a conserved structural motif which--like the zinc finger and leucine zipper--defines a family of related transcription factors. Homologies among POU sequences suggest a bipartite structure, consisting of an N-terminal POU-specific subdomain and C-terminal variant homeodomain connected by a linker of variable length and sequence. As a first step toward a molecular understanding of the Oct-2 POU domain and its mechanism of DNA recognition, we have overexpressed in Escherichia coli the intact POU domain and subdomains as thrombin-cleavable fusion proteins and have purified these fragments to homogeneity following digestion with thrombin. Biochemical and biophysical characterization yields the following results. (i) The intact POU domain (166 residues) is monomeric and exhibits high-affinity octamer-specific DNA-binding activity. (ii) Limited proteolytic digestion demonstrates that the POU domain contains two proteolytically stable subdomains (the POU-specific subdomain and the variant homeodomain) connected by a proteolytically sensitive linker. (iii) The isolated subdomains are each monomeric and do not interact to form noncovalent heterodimers.(ABSTRACT TRUNCATED AT 250 WORDS)

Solution structure of a POU-specific homeodomain: 3D-NMR studies of human B-cell transcription factor Oct-2

The POU DNA-binding motif defines a conserved family of eukaryotic transcription factors involved in regulation of gene expression. This bipartite motif consists of an N-terminal POU-specific domain (POUs), a flexible linker, and a C-terminal POU-specific homeodomain (POUHD). Here we describe the solution structure of a POU-specific homeodomain. An NMR model is obtained from Oct-2, a human B-cell specific transcription factor which participates in the regulation of immunoglobulin genes. A fragment of Oct-2 containing POUHD and an adjoining linker was expressed in Escherichia coli and characterized by three-dimensional nuclear magnetic resonance (3D-NMR) spectroscopy. Complete 1H and 15N resonance assignment of the POUHD moiety is presented. The POUHD solution structure, as calculated by distance geometry and simulated annealing (DG/SA), is similar to that of canonical homeodomains. A salient difference between solution and crystal structures is observed in the C-terminal segment of alpha-helix 3 (the HTH recognition helix), which is not well ordered in solution. Because this segment presumably folds upon specific DNA binding, its flexibility in solution may reduce the intrinsic DNA affinity of POUHD in the absence of POUs.

The construction of a cysteine-less melibiose carrier from E. coli

The melibiose carrier of E. coli is a cation-sugar cotransport system. This membrane protein contains four cysteine residues and the transport function is inhibited by sulfhydryl reagents. In order to investigate the importance of the cysteines, we have constructed a set of four melibiose transporters each of which has one cysteine replaced with serine or valine. The sensitivity of this set of carriers to N-ethylmaleimide was tested and Cys364 was identified as the target of the reagent. In addition, we constructed a melibiose transporter in which all 4 cysteines were replaced with either serine (Cys110, Cys310, and Cys364) or valine (Cys235) and we found that, as expected, the resulting cysteine-less transporter was resistant to the action of N-ethylmaleimide. The cysteine-less melibiose carrier had no significant decrease in ability to accumulate melibiose with cotransported sodium ions or protons. Thus, none of the 4 cysteines are necessary for the function of the melibiose carrier.

An efficient synthesis of a 4'-phosphonodifluoromethyl-3'-formyl-phenylalanine containing SRC SH2 ligand

A CuBr-mediated, regioselective cross-coupling between methyl 2,5-diiodobenzoate (4) and [(diethoxyphosphinyl)difluoromethyl]zinc bromide is reported. Palladium-catalyzed incorporation of an amino acid side chain, followed by subsequent modifications resulted in the rapid construction of 2. Compound 2 was designed to engage Cys188 of the Src SH2 domain, however, this was not observed spectroscopically.

Crystal structure of calcium-free human sorcin: a member of the penta-EF-hand protein family

Sorcin is a 22 kD calcium-binding protein that is found in a wide variety of cell types, such as heart, muscle, brain and adrenal medulla. It belongs to the penta-EF-hand (PEF) protein family, which contains five EF-hand motifs that associate with membranes in a calcium-dependent manner. Prototypic members of this family are the calcium-binding domains of calpain, such as calpain dVI. Full-length human sorcin has been crystallized in the absence of calcium and the structure determined at 2.2 A resolution. Apart from an extended N-terminal portion, the sorcin molecule has a globular shape. The C-terminal domain is predominantly alpha-helical, containing eight alpha-helices and connecting loops incorporating five EF hands. Sorcin forms dimers through the association of the unpaired EF5, confirming this as the mode of association in the dimerization of PEF proteins. Comparison with calpain dVI reveals that the general folds of the individual EF-hand motifs are conserved, especially that of EF1, the novel EF-hand motif characteristic of the family. Detailed structural comparisons of sorcin with other members of PEF indicate that the EF-hand pair EF1-EF2 is likely to correspond to the two physiologically relevant calcium-binding sites and that the calcium-induced conformational change may be modest and localized within this pair of EF-hands. Overall, the results derived from the structural observations support the view that, in sorcin, calcium signaling takes place through the first pair of EF-hands.

Bipartite DNA recognition by the human Oct-2 POU domain: POUs-specific phosphate contacts are analogous to those of bacteriophage lambda repressor

The POU motif defines a family of eukaryotic transcription factors broadly involved in tissue-specific gene expression and developmental regulation. The motif contains two DNA-binding domains: an N-terminal POU-specific domain (POUs) and C-terminal homeodomain (POUHD). Surprisingly, POUs has recently been found to be similar in structure to helix-turn-helix (HTH) domains of phage repressor and Cro proteins [Assa-Munt, N., Mortishire-Smith, R., Aurora, R., Herr, W., & Wright, P.E. (1993) Cell 73, 193-205; Dekker, N., Cox, M., Boelens, R., Verrijzer, C.P., van der Vliet, P.C., & Kaptein, R. (1993) Nature 362, 852-855]. Because POUHD and POUs are expected to bind DNA differently, we have used "methylphosphonate interference" to investigate the alignment of their HTH elements in a specific DNA complex. This neutral phosphate analogue, originally developed for applications in antisense drug design [Miller, P. S., & Ts'o, P. O. P. (1987) Anti-Cancer Drug Des. 2, 117-128], is shown to provide a sensitive probe for sites of backbone-specific protein-DNA interaction. Inferred POUs-phosphate contacts are in striking accord with cocrystal structures of bacteriophage repressor and Cro proteins. Alignment of POUHD and POUs in successive major grooves in each case predicts unique HTH-adenine contacts. This prediction is verified using DNA base analogues to effect interchange of AT functional groups by the method of 2'-deoxyinosine/5-methyl-2'-deoxycytosine substitution [McLaughlin, L. W., Benseler, F., Graeser, E., Piel, N., & Scholtissek, S. (1987) Biochemistry 26, 7238-7245]. Our results strongly support the hypothesis that the DNA-binding properties of POUs are analogous to those of bacteriophage lambda repressor.

An altered-specificity mutation in a human POU domain demonstrates functional analogy between the POU-specific subdomain and phage lambda repressor

The POU motif, conserved among a family of eukaryotic transcription factors, contains two DNA-binding domains: an N-terminal POU-specific domain (POUS) and a C-terminal homeodomain (POUHD). Surprisingly, POUS is similar in structure to the helix-turn-helix domains of bacteriophage repressor and Cro proteins. Such similarity predicts a common mechanism of DNA recognition. To test this prediction, we have studied the DNA-binding properties of the human Oct-2 POU domain by combined application of chemical synthesis and site-directed mutagenesis. The POUS footprint of DNA contacts, identified by use of modified bases, is analogous to those of bacteriophage repressor-operator complexes. Moreover, a loss-of-contact substitution in the putative POUS recognition alpha-helix leads to relaxed specificity at one position in the DNA target site. The implied side chain-base contact is identical to that of bacteriophage repressor and Cro proteins. These results establish a functional analogy between the POUS and prokaryotic helix-turn-helix elements and suggest that their DNA specificities may be governed by a shared set of rules.

Mapping critical residues in eukaryotic DNA-binding proteins: a plasmid-based genetic selection strategy with application to the Oct-2 POU motif

Discrimination between allowed and disallowed amino acid substitutions provides a powerful method for analysis of protein structure and function. Site-directed mutagenesis allows specific hypotheses to be tested, but its systematic application to entire structural motifs is inefficient. This limitation may be overcome by genetic selection, which allows rapid scoring of thousands of randomly (or pseudorandomly) generated mutants. To facilitate structural dissection of DNA-binding proteins, we have designed two generally applicable bacterial selection systems: pPLUS selects for the ability of a protein to bind to a user-defined DNA sequence, whereas pMINUS selects against such binding. Complementary positive and negative selections allow rapid mapping of critical residues. To test and calibrate the systems, we have investigated the bipartite POU domain of the human B-cell-specific transcription factor Oct-2. (i) An invariant residue (Asn347) in the DNA-recognition helix of the POU-specific homeodomain (POUHD) was substituted by each of the 19 other possible amino acids. The mutant proteins each exhibited decreased specific DNA binding as defined in vivo by genetic selection and in vitro by gel retardation; relative affinities correlate with phenotypes in the positive and negative selection systems. An essential role for Asn347 in wild-type POUHD-DNA recognition is consistent with homologous Asn-adenine interactions in cocrystal structures of canonical homeodomains. (ii) Extension of pPLUS/pMINUS selection to the POU-specific subdomain (POUs) is demonstrated by analysis of mutations in its putative helix-turn-helix (HTH) element.(ABSTRACT TRUNCATED AT 250 WORDS)

Sugar recognition mutants of the melibiose carrier of Escherichia coli: possible structural information concerning the arrangement of membrane-bound helices and sugar/cation recognition site

Melibiose carrier mutants, isolated by growing cells on melibiose plus the non-metabolizable competitive inhibitor thiomethyl-beta-galactoside (TMG), were studied to determine sugar and cation recognition abnormalities. Most of the mutants show good transport of melibiose but have lost the recognition of TMG. In addition, most mutants show little or no transport of lactose. Cation recognition is also affected as all of these mutants have lost the ability to transport protons with melibiose. The amino acids causing these mutations were determined by sequencing the melB gene on the plasmid. The mutations were located on helices I, IV, VII, X and XI. We propose that these five helices are in proximity with each other and that they line the sugar/cation transport channel.

Selective inhibition of Src SH2 by a novel thiol-targeting tricarbonyl-modified inhibitor and mechanistic analysis by (1)H/(13)C NMR spectroscopy

Detailed analysis of Src SH2 binding by peptides containing a novel tricarbonyl-modified pTyr moiety is described. We envisaged that Src SH2 selectivity might be obtained by exploiting the thiol group of Cys188 present in the pTyr binding pocket of the protein at the betaC3 position. Peptidyl as well as non-peptidyl compounds 1-4 possessing a 4-alpha,beta-diketoester-modified pTyr mimic exhibited micromolar affinity to Src SH2. Furthermore, these tricarbonyl compounds were selective for Src SH2 to the extent they showed no significant affinity for either Cys188Ser or Cys188Ala Src SH2 mutants. Upon closer examination of the binding of these tricarbonyls to Src SH2 using NMR of 13C-labeled compounds (6a, 6b, and 6c), we found that after the initial binding event the molecule disproportionated in a 'retro-Claisen' fashion to provide benzoic acid 16 and, following hydrolysis of the methyl ester 17, the hemiketal adduct of glyoxalic acid 18.

An invariant asparagine in the POU-specific homeodomain regulates the specificity of the Oct-2 POU motif

The homeodomain defines a family of transcription factors broadly involved in the regulation of gene expression. DNA recognition, as observed in three representative complexes (Engrailed, Antennapedia, and MAT alpha 2), is mediated in the major groove by a helix-turn-helix (HTH) element and in the minor groove by an N-terminal arm. The three complexes share similar overall features, but they also exhibit significant differences in DNA interactions. Because these differences may distinguish the biological activities of different classes of homeodomains, we have investigated the contribution of the Oct-2 POU-specific homeodomain (POUHD) to the specificity of the bipartite POU motif. Comparative studies of variant protein-DNA complexes demonstrate the following. (i) Mutations in an invariant residue in the POUHD HTH (N347; residue 10 of the putative recognition alpha-helix) reduce octamer binding with the relaxation of specificity at one position (5'-ATGCAAAT). The inferred HTH side chain-base interaction, although not observed in the solution structure of the Antennapedia complex, is in accord with homologous contacts in the Engrailed and MAT alpha 2 cocrystal structures. (ii) Comparison of the DNA-binding properties of POU and POUHD demonstrates that POUs and POUHD independently regulate specificity at opposite ends of the DNA site (5'-TATGCAAAT). Both domains contact the two central bases (5'-TATGCAAAT) where coordinate binding of POUS in the major groove overrides the intrinsic specificity of POUHD in the minor groove. (iii) The differential sensitivity of POU and POUHD to 2'-deoxyinosine substitutions (a minor-groove modification) suggests that POUS binding repositions the POUHD N-terminal "arm".(ABSTRACT TRUNCATED AT 250 WORDS)

Carboxyl-terminal truncations of the melibiose carrier of Escherichia coli

The melibiose carrier of Escherichia coli is predicted to possess a short NH2 terminus, 11 transmembrane segments joined by short hydrophilic regions, and a 40-residue hydrophilic carboxyl terminus of unknown function. This paper describes truncations of the carboxyl terminus at eight locations using site-specific mutagenesis to introduce stop codons. Measurement of sugar transport and cation-coupling characteristics indicate that the carboxyl tail plays no direct role in substrate recognition or energy transduction. Thirty-six amino acids could be removed from the hydrophilic carboxyl domain without the loss of sugar specificity, facilitated diffusion, uphill transport, H+-coupling or Na+-coupling characteristics. These results are consistent with the hypothesis that the sugar/cation binding site is formed by the interaction of the transmembrane helices 3, 4, 6, 9, and 10 and does not involve the carboxyl-terminal portion of the protein. When truncations were made within the hydrophobic domain of transmembrane helix 11 (truncations of 41 or more residues), the carrier was no longer found in the membrane. This suggests that the carboxyl terminus may be involved in the membrane insertion process, stabilization of the carrier within the membrane following insertion, or protection of the inserted carrier from proteolytic scavenging. A new plasmid that expresses the temperature-resistant isoform of the melibiose carrier under inducible control of a tac promoter, designated pKKMB, is also described.

Peptide-specific antibody for the melibiose carrier of Escherichia coli localizes the carboxyl terminus to the cytoplasmic face of the membrane

The synthetic decapeptide NH2-Cys-Val-Gly-Ala-Val-Ser-Asp-Val-Lys-Ala-COOH (designated MBct10), which corresponds to the carboxyl terminus of the melibiose carrier of Escherichia coli, was synthesized and used to raise antibodies in a rabbit. Anti-MBct10 antibodies recognizes the normal melibiose carrier but not a truncated carrier lacking 14 carboxyl-terminal amino acids. Thus the antibodies are specific for the carboxyl terminus of the carrier and not for other domains of the protein. When right-side-out and inside-out membrane vesicles were probed with anti-MBct10 serum, only the inside-out vesicles bound antibody. The carboxyl terminus of the melibiose carrier protein is therefore exposed on the cytoplasmic surface of the membrane. The co-localization of both NH2- and carboxyl termini to the cytoplasmic surface dictates that the protein cross the membrane an even number of times. These data together with hydrophobicity analysis support a topological model for the melibiose carrier with 10 or 12 transmembrane domains.

Mutations that simultaneously alter both sugar and cation specificity in the melibiose carrier of Escherichia coli

The isolation and deduced amino acid sequence of 70 melibiose carrier mutants with impaired methyl-beta-D-galactopyranoside (TMG) and cation recognition properties is described. The Km for TMG transport ranged from 1 to greater than 100 mM. Amino acid substitutions occurred at 23 unique sites within the protein. These sites were clustered into four distinct regions: Asp-15 through Ile-18 (cluster I), Tyr-116 through Pro-122 (cluster II), Val-342 through Ile-348 (cluster III), and Ala-364 through Gly-374. Only two sites fell outside of these clusters: Ile-61 and Ala-236. In the native conformation, some or all of these clusters may interact to form the substrate recognition site. Impairment of TMG recognition was accompanied by decreased Li+ inhibition of melibiose transport in all but one mutant. That changes in sugar recognition properties should so frequently accompany changes in cation recognition properties suggests an interaction between the two substrates. A model for such interaction is proposed.