Two regions of mature periplasmic maltose-binding protein of Escherichia coli involved in secretion

"Fishing and Hunting"-Selective Immobilization of a Recombinant Phenylalanine Ammonia-Lyase from Fermentation Media

This article overviews the numerous immobilization methods available for various biocatalysts such as whole-cells, cell fragments, lysates or enzymes which do not require preliminary enzyme purification and introduces an advanced approach avoiding the costly and time consuming downstream processes required by immobilization of purified enzyme-based biocatalysts (such as enzyme purification by chromatographic methods and dialysis). Our approach is based on silica shell coated magnetic nanoparticles as solid carriers decorated with mixed functions having either coordinative binding ability (a metal ion complexed by a chelator anchored to the surface) or covalent bond-forming ability (an epoxide attached to the surface via a proper linker) enabling a single operation enrichment and immobilization of a recombinant phenylalanine ammonia-lyase from parsley fused to a polyhistidine affinity tag.

Amylosucrase-mediated synthesis and self-assembly of amylose magnetic microparticles

Magnetic separation and purification of MBP-tagged protein using AMB.

Superfolder GFP is fluorescent in oxidizing environments when targeted via the Sec translocon

The ability to study proteins in live cells using genetically encoded fluorescent proteins (FPs) has revolutionized cell biology (1-3). Researchers have created numerous FP biosensors and optimized FPs for specific organisms and subcellular environments in a rainbow of colors (4,5). However, expressing FPs in oxidizing environments such as the eukaryotic endoplasmic reticulum (ER) or the bacterial periplasm can impair folding, thereby preventing fluorescence (6,7). A substantial fraction of enhanced green fluorescent protein (EGFP) oligomerizes to form non-fluorescent mixed disulfides in the ER (6) and EGFP does not fluoresce in the periplasm when targeted via the SecYEG translocon (7). To overcome these obstacles, we exploited the highly efficient folding capability of superfolder GFP (sfGFP) (8). Here, we report sfGFP does not form disulfide-linked oligomers in the ER and maltose-binding protein (MBP) signal sequence (peri)-sfGFP (9) is brightly fluorescent in the periplasm of Escherichia coli. Thus, sfGFP represents an important research tool for studying resident proteins of oxidizing environments.

Protein quality control in the bacterial periplasm

The proper functioning of extracytoplasmic proteins requires their export to, and productive folding in, the correct cellular compartment. All proteins in Escherichia coli are initially synthesized in the cytoplasm, then follow a pathway that depends upon their ultimate cellular destination. Many proteins destined for the periplasm are synthesized as precursors carrying an N-terminal signal sequence that directs them to the general secretion machinery at the inner membrane. After translocation and signal sequence cleavage, the newly exported mature proteins are folded and assembled in the periplasm. Maintaining quality control over these processes depends on chaperones, folding catalysts, and proteases. This article summarizes the general principles which control protein folding in the bacterial periplasm by focusing on the periplasmic maltose-binding protein.

Signal sequence and alanine-rich region of streptococcal protein antigen A of Streptococcus sobrinus can direct localization of alkaline phosphatase to the periplasm of Escherichia coli

Streptococcal protein antigen A (SpaA) of Streptococcus sobrinus is expressed on the surface of cells and extracellularly. TnphoA which lacks signals for transcription and membrane transport of Escherichia coli alkaline phosphatase was used to analyze the sequences necessary for transport of a SpaA/PhoA fusion protein across the cytoplasmic membrane to the periplasm of E. coli cells. Of 15 alkaline phosphatase-producing isolates analyzed, all were found to localize more than 85% of the SpaA/PhoA hybrid protein to the periplasm of E. coli cells. From DNA sequence analysis, all were found to have TnphoA inserted into an identical site. The insertion site of TnphoA was downstream from the coding sequence that generates four tandemly repeated alanine-rich sequences of 82 amino acid residues. These results suggest that in addition to the signal sequence, mature protein sequences containing alanine-rich repeat sequences may play a role in the export of the SpaA protein across a bacterial membrane.

Improved expression characteristics of single-chain Fv fragments when fused downstream of the Escherichia coli maltose-binding protein or upstream of a single immunoglobulin-constant domain

The expression of single-chain Fv fragments (scFv) targeted to the periplasm of Escherichia coli often results in very low yields of soluble protein frequently accompanied by host cell growth arrest and sometimes lysis. Single-chain antibody fragments (scAb) are scFv with a human kappa light chain constant (HuCkappa) domain attached C-terminally and share similar problems of expression. By fusing the E. coli maltose-binding protein (mbp) gene either 3' or 5' to a scAb specific for the herbicide atrazine, a reduction in growth arrest was observed that was dependent on the order of gene fusion. The scAb-mbp fusion delayed the onset of growth arrest following induction while the mbp-scAb fusion appeared to ablate growth arrest completely. Cell fractionation revealed barely detectable levels of scAb-mbp in the periplasm while mbp-scAb was detected at equivalent levels as scAb in the periplasmic compartment, indicating that periplasmic scAb solubility is unrelated to propensity to cause growth arrest. IMAC purification of scAb and mbp-scAb proteins followed by liquid competition ELISA revealed the IC(50) for atrazine to be approximately 1 nM for both proteins demonstrating that 5'-mbp fusion does not alter antigen binding. The equivalent scFv and mbp-scFv vectors expressed far less material in both periplasmic and insoluble fractions indicating that the HuCkappa domain can have a positive effect on scFv expression when expressed either alone or as a mbp fusion. The ablation of growth arrest by a 5'-mbp fusion and enhancement of expression by a 3'-HuCkappa domain fusion were extended to a second scFv specific for the herbicide diuron. Therefore, by expressing scFv as tripartite fusions (mbp-scFv-HuCkappa) enhanced levels of soluble periplasmic expression can be achieved without causing growth arrest of the host cell, realizing the potential for constitutive expression of hapten-binding scFv in the E. coli periplasm.

Activity of protein MalE (maltose-binding protein) fused to cytoplasmic and periplasmic regions of an Escherichia coli inner membrane protein

We analysed the properties of mature MBP (maltose-binding protein or MalE protein) fused to an integral cytoplasmic membrane protein of Escherichia coli. Fusion of MalE to the first MalG periplasmic loop enabled a strain defective in the malE gene to utilize maltose. In contrast, fusion of MalE to a cytoplasmic loop did not complement the malE delta 444 deletion. We obtained results highly correlated with those obtained by using alkaline phosphatase as a reporter for the topology of MalG. We discuss the possibility of genetically determining the topology of cytoplasmic membrane proteins by a method based on engineered fusions to MBP.

Folding of a mutant maltose-binding protein of Escherichia coli which forms inclusion bodies

The maltose-binding protein (MalE) of Escherichia coli is the periplasmic component of the transport system for malto-oligosaccharides. We have examined the characteristics of a Mal- mutant of malE corresponding to the double substitution Gly32 --> Asp/Ile33 --> Pro, MalE31, previously obtained by random mutagenesis. In vivo, the MalE31 precursor is efficiently processed, but the mature protein forms inclusion bodies in the periplasm. Furthermore, the accumulation of insoluble MalE31 is independent of its cellular localization; MalE31 lacking its signal sequence forms inclusion bodies in the cytoplasm. The native MalE31 protein can be purified by affinity chromatography from inclusion bodies after denaturation by 8 M urea. The renatured protein exhibits full maltose binding affinity (Kd= 9 x 10(-7) M), suggesting that its folded structure is similar to that of the wild-type protein. Unfolding/refolding experiments show that MalE31 is less stable (-5. 5 kcal/mol) than the wild-type protein (-9.5 kcal/mol) and that folding intermediates have a high tendency to form aggregates. In conclusion, the observed phenotype of cells expressing malE31 can be explained by a defective folding pathway of the protein.

Thirty-three amino acids of the mature moiety of an unprocessed maltose-binding protein are sufficient for export in Escherichia coli

Maltose-binding protein (MBP) is translocated across the cytoplasmic membrane of Escherichia coli; successful export depends on information in both the signal peptide and the mature moiety of the protein. To determine the shortest portion of the mature region that would maintain detectable entry of MBP into the export pathway, we took advantage of the properties of an MBP species with proline substituted in the +1 position relative to the cleavage site (MBP27-P). This protein efficiently crosses the cytoplasmic membrane but is not processed and acts as a competitive inhibitor of signal peptidase I (leader peptidase). Export of MBP27-P is measured by the inhibition of processing of other proteins, such as ribose-binding protein (RBP). A series of truncated derivatives of MBP27-P were tested for the ability to inhibit processing of RBP. An MBP27-P species with only 33 amino acids of the mature moiety inhibited processing of RBP, indicating that this truncated polypeptide was probably exported and interacted with signal peptidase I.

Protein secretion in Bacillus species

Bacilli secrete numerous proteins into the environment. Many of the secretory proteins, their export signals, and their processing steps during secretion have been characterized in detail. In contrast, the molecular mechanisms of protein secretion have been relatively poorly characterized. However, several components of the protein secretion machinery have been identified and cloned recently, which is likely to lead to rapid expansion of the knowledge of the protein secretion mechanism in Bacillus species. Comparison of the presently known export components of Bacillus species with those of Escherichia coli suggests that the mechanism of protein translocation across the cytoplasmic membrane is conserved among gram-negative and gram-positive bacteria differences are found in steps preceding and following the translocation process. Many of the secretory proteins of bacilli are produced industrially, but several problems have been encountered in the production of Bacillus heterologous secretory proteins. In the final section we discuss these problems and point out some possibilities to overcome them.

Secretion of mutant leucine-specific binding proteins with internal deletions in Escherichia coli

The leucine-specific binding protein, encoded by the livK gene, is located in the periplasm of E. coli. The present study is an attempt to identify intragenic regions that determine the efficiency of its secretion into the periplasm. C-terminal deletions or fusions of the livK gene to trpA (encoding the alpha subunit of tryptophan synthetase) were secreted with little loss of efficiency [1]. A series of deletions was constructed at the unique Sphl site within livK, near the 5' end of the region coding for the mature protein. Between 16 and 113 amino acids were deleted in the amino-terminal one-third of the protein. A few of these deletions were located within a few amino acids of the signal sequence processing site. Deletions extending within thirteen residues of the processing site were processed and secreted more slowly than normal. Secondary structure predictions suggested that the alpha-helical core region of the signal sequence extends into the mature protein in the case of the slow processing mutants, perhaps interfering with the recognition site for leader peptidase or other secretory components. These results suggest that the conformation around the signal processing site may be a critical factor in determining the efficiency of secretion. During the course of this study, it was found that the difference in molecular weight between precursor and mature forms of some binding protein mutants, as judged by SDS-PAGE, was much greater than could be accounted for by processing of the signal sequence. This anomalous mobility on gels, however, could be eliminated by performing SDS-PAGE in the presence of 6 M urea.

Genetic approach to the role of tryptophan residues in the activities and fluorescence of a bacterial periplasmic maltose-binding protein

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 role of the mature part of secretory proteins in translocation across the plasma membrane and in regulation of their synthesis in Escherichia coli

Presently available data are reviewed which concern the role of the mature parts of secretory precursor proteins in translocation across the plasma membrane of Escherichia coli. The following conclusions can be drawn; i) signals, acting in a positive fashion and required for translocation do not appear to exist in the mature polypeptides; ii) a number of features have been identified which either affect the efficiency of translocation or cause export incompatibility. These are: alpha) protein folding prior to translocation; beta) restrictions regarding the structure of N-terminus; gamma) presence of lipophilic anchors; delta) too low a size of the precursor. Efficiency of translocation is also enhanced by binding of chaperonins (SecB, trigger factor, GroEL) to precursors. Binding sites for chaperonins appear to exist within the mature parts of the precursors but the nature of these sites has remained rather mysterious. Mutant periplasmic proteins with a block in release from the plasma membrane have been described, the mechanism of this block is not known. The mature parts of secretory proteins can also be involved in the regulation of their synthesis. It appears that exported proteins are already recognized as such before they are channelled into the export pathway and that their synthesis can be feed-back inhibited at the translational level.