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The periplasmic maltose binding protein, MalE, is a major element in maltose transport and in chemotaxis towards this sugar. Previous genetic analysis of the MalE protein revealed functional domains involved in transport and chemotactic functions. Among them the surface located alpha helix 7, which is part of the C-lobe, one of the two lobes forming the three dimensional structure of MalE. Small deletions in this region abolished maltose transport, although maintaining wild-type affinity and specificity as well as a normal chemoreceptor function. It was suggested that alpha helix 7 may be implicated in interactions between the maltose binding protein and the membrane-bound protein complex (Duplay P, Szmelcman S. 1987. Silent and functional changes in the periplasmic maltose binding protein of Escherichia coli K12. II. Chemotaxis towards maltose. J Mol Biol 194:675-678: Duplay P, Szmelcman S, Bedouelle H, Hofnung M. 1987. Silent and functional changes in the periplasmic maltose binding protein of Escherichia coli K12. I: Transport of maltose. J Mol Biol 194:663-673). In this study, we submitted a region of 14 residues--Asp 207 to Gly 220--encompassing alpha helix 7, to genetic analysis by oligonucleotide mediated random mutagenesis. Out of 127 identified mutations, twelve single and five double mutants with normal affinities towards maltose were selected for further investigation. Two types of mutations were characterized, silent mutations that did not affect maltose transport and mutations that heavily impaired transport kinetics, even thought the maltose binding capacity of the mutant proteins remained normal. Three substitutions at Tyr 210 (Y210S, Y210L, Y210N) drastically reduced maltose transport. One substitution at Ala 213 (A213I) and one substitution at Glu 214 (E214K) also impaired transport. These three identified residues, Tyr 210, Ala 213, and Glu 214, which are constituents of alpha helix 7, therefore seem to play some important role in maltose transport, most probably in a productive interaction between the MalE protein and the membrane bound MalFGK2 complex.

The X-ray structures of the maltose bound forms of two insertion/deletion mutants of the Escherichia coli maltodextrin binding protein, MalE322 and MalE178, have been determined and refined. MalE322 involves a one residue deletion, two residue insertion in a hinge segment connecting the two (N and C) domains of the protein, an area already identified as being critical for the correct functioning of the protein. MalE178 involves a nine residue deletion and two residue insertion in a helix at the periphery of the C-domain. The function of both mutant proteins is similar to the wild-type, although MalE322 increases the ability to transport maltose and maltodextrin whilst inhibiting the ability of the cell to grow on dextrins. Both proteins exhibit very localized and conservative conformational changes due to their mutations. The structure of MalE322 shows some deformation of the third hinge strand, indicating the likely cause of change in its biochemistry. MalE178 is stable and its activity virtually unchanged from the wild-type. This is most likely due to the long distance of the mutation from the binding site and conservation of the number of interactions between the area around the deletion site and the main body of the protein.

The construction of recombinant proteins by genetic engineering has opened new avenues in basic research (studies on protein organization, protein folding, immunogenicity of proteins, ...) and many different applications. Recombinant proteins which keep properties of both parental proteins are especially interesting. For example, if one protein--the vector protein--is targeted to a given cellular compartment, the other protein--the passenger--may be identically targeted. Also, if the vector protein can be purified by a simple affinity chromatographic procedure, this property may be extended to the passenger. The authors have developed a genetic procedure to detect "permissive" sites within potential vector proteins so that genetic fusion to these sites keep most or all biological properties of the vector. When they used LamB, an outer membrane protein from E. coli, foreign sequences could be expressed at the bacterial cell surface. This may lead to several types of applications: live bacterial vaccines, simple diagnostic tests, selection procedures for peptides with biological activity. When they used the MalE protein, a periplasmic maltose binding protein from E. coli, the passengers could be exported and purified in one-step high affinity chromatography in mild non-denaturing conditions. This led us to a simple preparation and purification scheme for the soluble part of the CD4 receptor for the Human Immunodeficiency Virus (HIV).

The periplasmic maltose-binding protein (MBP or MalE protein) of Escherichia coli is an essential element in the transport of maltose and maltodextrins and in the chemotaxis towards these sugars. On the basis of previous results suggesting their possible role in the activity and fluorescence of MBP, we have changed independently to alanine each of the eight tryptophan residues as well as asparagine 294, which is conserved among four periplasmic sugar-binding proteins. Five of the tryptophan mutations affected activity. In four cases (substitution of Trp62, Trp230, Trp232 and Trp340), there was a decrease in MBP affinity towards maltose correlated with modifications in transport and chemotaxis. According to the present state of the 2.3 A three-dimensional structure of MBP, all four residues are in the binding site. Residues Trp62 and Trp340 are in the immediate vicinity of the bound substrate and appear to have direct contacts with maltose; this is in agreement with the drastic increases in Kd values (respectively 67 and 300-fold) upon their substitution by alanine residues. The modest increase in Kd (12-fold) observed upon mutation of Trp230 would be compatible with the lesser degree of interaction this residue has with the bound substrate and the idea that it plays an indirect role, presumably by keeping other residues involved directly in binding in their proper orientation. Substitution of Trp232 resulted in a small increase in Kd value (2-fold) in spite of the fact that this residue is the closest to the ligand of the tryptophan residues according to the three-dimensional model. In the fifth case, replacement of Trp158, which is distant from the binding site, strongly reduced the chemotactic response towards maltose without affecting the transport parameters or the sugar-binding activities of the mutant protein. Trp158 may therefore be specifically implicated in the interaction of MBP with the chemotransducer Tar, but this effect is likely to be indirect, since Trp158 is buried in the structure of MBP. Of course, some structural rearrangements could be responsible in part for the effects of these mutations. The remaining four mutations were silent. The corresponding residues (Trp10, Trp94, Trp129 and Asn294) are all distant from the sugar-binding site on the crystallographic model of MBP, which is in agreement with their lack of effect on binding. In addition, our results show that they play no role in the interactions with the other proteins of the maltose transport (MalF, MalG or MalK) or chemotaxis (Tar) systems.(ABSTRACT TRUNCATED AT 400 WORDS)

The 177 N-terminal amino acids of CD4, the receptor of the human immunodeficiency virus (HIV), have been expressed in Escherichia coli as genetic fusions to the periplasmic maltose-binding protein (MalE) from this organism. A large fraction of the hybrid proteins can be released from the periplasm by osmotic shock and purified in one step on a cross-linked amylose column eluted with maltose under mild conditions. One hybrid protein binds HIV envelope protein gp160 and neutralizes the virus in vitro. This provides the first example of the production and one-step purification of an active form of an eukaryotic protein by fusion to MalE. The use of this system for mass screening of CD4 mutants, high-scale production of the hybrid protein for structural studies on CD4, testing antiviral compounds, and therapeutic assays is discussed.

Several hundred copies of a highly conserved extragenic palindromic sequence, 20-40 nucleotides long, exist along the chromosome of E. coli and S. typhimurium. These have been defined as palindromic units (PU) or repetitive extragenic palindromes (REP). No general function for PUs has been identified. In the present work, we provide data showing that a protein associated with a chromoid extract of E. coli protects PU DNA against exonuclease III digestion. This provides the first experimental evidence that PU constitutes binding sites for a chromoid-associated protein. This result supports the hypothesis that PUs could play a role in the structure of the bacterial chromoid.

Sensory transduction in bacterial chemotaxis is beginning to be understood at the molecular level. At the receptor end, we have some considerable knowledge about the molecular properties of chemoreceptors. At the effector end, we know that flagella rotate and that the direction of rotation is determined by attractants and repellents, although we do not yet know the molecular features of the motor and the gear shift. Between the receptors and the effectors is a system for integrating the sensory transduction, which somewhow involves methylation of membrane proteins and possibly a change in membrane potential, but further details of how the mechanism works remain to be elucidated. It seems to us likely that the facts and concepts learned from a study of sensory transduction in bacteria can be applied to answering questions about transduction mechanisms in eukaryotic cells. Examples include the following: How do sensory stimuli produce their effects in sensory receptor cells? How do neurotransmitters act at receptors of postsynaptic cells to produce the variety of effects possible (changes in membrane potential, in secretion, in contraction, etc.)? How do hormones interact with their receptors to bring about various responses?

The malE gene encodes the periplasmic maltose-binding protein (MBP). Nineteen mutations that still permit synthesis of stable MBP were generated by random insertion of a BamHI octanucleotide into malE and six additional mutations by in-vitro recombinations between mutant genes. The sequence changes were determined; in most cases the linker insertion is accompanied by a small deletion (30 base-pairs on average). The mutant MBP were studied for export, growth on maltose and maltodextrins, maltose transport and binding, and maltose-induced fluorescence changes. Sixteen mutant MBP (out of 21 studied in detail) were found in the periplasmic space: 12 of them retained a high affinity for maltose, and 10 activity for growth on maltose. The results show that several regions of MBP are dispensable for stability, substrate binding and export. Three regions (residues 207 to 220, 297 to 303 and 364 to 370) may be involved in interactions with the MalF or MalG proteins. A region near the C-terminal end is important for maltose binding. Two regions of the mature protein (residues 18 to 42 and 280 to 296) are required for export to, or solubility in, the periplasm.

The kinetic parameters for the maltose transport system in Escherichia coli K12 were determined with maltose and maltotriose as substrates. The system exhibits an apparent Km of 1 muM for maltose and 2 muM for maltotriose. The V of entry was determined as 2.0 and 1.1 nmol substrate/min per 10(8) cells. Mutations in lamB, the structural gene for the receptor protein of phage lambda, increased the Km for maltose transport by a factor of 100-500 without influencing the maximal rate of transport. Maltotriose is no longer transported in these lamB mutants. The maltose-binding protein, an essential component of the maltose transport system, was found to exhibit substrate-dependent fluorescence quenching. This phenomenon was used to determine dissociation constants and to estimate the rate of ligand dissociation. A Kd of 1 muM for maltose and of 0.16 muM for maltotroise was found. From the comparison of the kinetic parameters of transport of maltose and maltotriose in wild-type and lambda-resistant mutants with the binding constants for both sugars to purified maltose-binding protein, we conclude that the lambda receptor facilitates the diffusion of maltose and maltodextrins through the outer membrane.

We examined the chemotactic behavior of ten Escherichia coli mutants able to synthesize a modified periplasmic maltose-binding protein (MBP) retaining high affinity for maltose. Eight were able to grow on maltose (Mal+), two were not (Mal-). In the capillary assay six out of eight of the Mal+ strains showed an optimal response at the same concentration of maltose as the wild-type strain; the amplitude of the response was strongly reduced in two Mal+ mutants and partially affected in one. The amplitude of the chemotactic response of the two Mal- strains was at least equal to that of the wild type, so that the chemotactic and transport functions of MBP were dissociated in these two cases. We define two regions of the protein (residues 297 to 303 and 364 to 369), that are important both for the chemotactic response and for transport, and one region (residues 207 to 220) that is essential for transport but dispensable for chemotaxis. Interestingly, some regions that were found to be inessential for transport are also dispensable for chemotaxis.

Genetic fusions allowing the expression in E. coli of hybrid proteins between a bacterial periplasmic maltose binding protein (MalE) and the CD4 molecule (the receptor of the HIV virus) have been constructed. One of them has kept most of the properties of each constituent: it is exported, can be purified in one step on an affinity column, interacts with anti-MalE and anti-CD4 antibodies, binds HIV gp 120 protein and inactivates HIV virus in an in vitro test.