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MYXO-CTERM sorting tag directs proteins to the cell surface via the type II secretion system.

ABSTRACT: Cells interact with their surrounding environment through surface proteins. However, knowledge gaps remain in understanding how these important types of proteins are transported and anchored on the cell surface. In the Gram-negative social bacterium, Myxococcus xanthus, a putative C-terminal sorting tag (MYXO-CTERM) is predicted to help direct 34 different proteins onto the cell surface. Here we investigate the sorting pathway for MYXO-CTERM proteins by using the TraA cell surface receptor as a paradigm. Deleting this motif from TraA abolishes the cell surface anchoring and results in extracellular secretion. Our findings indicate that conserved cysteines within the MYXO-CTERM are posttranslationally modified and are required for TraA cell surface localization and function. A region immediately upstream of these residues is predicted to be disordered and removing this motif caused a secretion defect and blocked cell surface anchoring. We further show that the type II secretion system is required for translocation across the outer membrane and that a cysteine-rich region directs TraA to the T2SS. Similar results were found with another MYXO-CTERM protein indicating our findings can be generalized. Further, we show the universal distribution of MXYO-CTERM motif across the Myxococcales order and provide a working model for sorting of these proteins.

SUBMITTER: Sah GP

PROVIDER: S-EPMC7237302 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:BACKGROUND:Heterologous gene expression is well established for various prokaryotic model systems. However, low yield, incorrect folding and instability still impede the production of soluble, bioactive proteins. To improve protein production with the Gram-positive host Bacillus subtilis, a secretory expression system was designed that enhances translocation, folding and stability of heterologous proteins, and simplifies purification. Based on the theta-replication plasmid pHT01, a B. subtilis secretory expression vector was constructed that encodes a fusion protein consisting of a signal peptide and a StrepII-tag linked to a SUMO-tag serving as a folding catalyst. The gene of a protein of interest can be translationally fused to the SUMO cassette and an additional 6xHis-tag encoding region. In order to maximize secretory expression of the construct by fitting the signal peptide to the StrepII-SUMO part of the fusion protein, a B. subtilis signal-peptide library was screened with the Escherichia coli alkaline phosphatase PhoA as a reporter. RESULTS:The YoaW signal peptide-encoding region (SPyoaW) was identified with highest secretory expression capacity in context with the StrepII-SUMO-tag fusion in a B. subtilis eightfold extracellular protease deletion strain. PhoA activity and fusion protein production was elevated by a factor of approximately five when compared to an ?-amylase (AmyQ) signal peptide construct. Replacement of PhoA with a single-chain variable fragment antibody specific for GFP or the B. amyloliquefaciens RNase barnase, respectively, resulted in a similar enhancement of secretory expression, demonstrating universality of the YoaW signal peptide-StrepII-SUMO encoding cassette for secretory expression in B. subtilis. Optimisation of codon usage and culture conditions further increased GFP-specific scFv fusion-protein production, and a simple affinity purification strategy from culture supernatant with removal of the StrepII-SUMO-tag by SenP-processing yielded 4 mg of pure, soluble and active GFP-specific scFv from 1 l of culture under standard laboratory conditions. CONCLUSIONS:The new expression system employing a YoaW signal peptide-StrepII-SUMO fusion will simplify secretory protein production and purification with B. subtilis. It can obviate the need for time consuming individual signal-peptide fitting to maximize yield for many different heterologous proteins of interest.

Project description:BACKGROUND: The discovery of the autotransporter family has provided a mechanism for surface expression of proteins in laboratory strains of Escherichia coli. We have previously reported the use of the AIDA-I autotransport system to express the Salmonella enterica serovar Enteritidis proteins SefA and H:gm. The SefA protein was successfully exposed to the medium, but the orientation of H:gm in the outer membrane could not be determined due to proteolytic cleavage of the N-terminal detection-tag. The goal of the present work was therefore to construct a vector containing elements that facilitates analysis of surface expression, especially for proteins that are sensitive to proteolysis or otherwise difficult to express. RESULTS: The surface expression system pAIDA1 was created with two detection tags flanking the passenger protein. Successful expression of SefA and H:gm on the surface of E. coli was confirmed with fluorescently labeled antibodies specific for the N-terminal His6-tag and the C-terminal Myc-tag. While both tags were detected during SefA expression, only the Myc-tag could be detected for H:gm. The negative signal indicates a proteolytic cleavage of this protein that removes the His6-tag facing the medium. CONCLUSIONS: Expression levels from pAIDA1 were comparable to or higher than those achieved with the formerly used vector. The presence of the Myc- but not of the His6-tag on the cell surface during H:gm expression allowed us to confirm the hypothesis that this fusion protein was present on the surface and oriented towards the cell exterior. Western blot analysis revealed degradation products of the same molecular weight for SefA and H:gm. The size of these fragments suggests that both fusion proteins have been cleaved at a specific site close to the C-terminal end of the passenger. This proteolysis was concluded to take place either in the outer membrane or in the periplasm. Since H:gm was cleaved to a much greater extent then the three times smaller SefA, it is proposed that the longer translocation time for the larger H:gm makes it more susceptible to proteolysis.

Project description:BACKGROUND: Protein translocation to the proper cellular destination may be guided by various classes of sorting signals recognizable in the primary sequence. Detection in some genomes, but not others, may reveal sorting system components by comparison of the phylogenetic profile of the class of sorting signal to that of various protein families. RESULTS: We describe a short C-terminal homology domain, sporadically distributed in bacteria, with several key characteristics of protein sorting signals. The domain includes a near-invariant motif Pro-Glu-Pro (PEP). This possible recognition or processing site is followed by a predicted transmembrane helix and a cluster rich in basic amino acids. We designate this domain PEP-CTERM. It tends to occur multiple times in a genome if it occurs at all, with a median count of eight instances; Verrucomicrobium spinosum has sixty-five. PEP-CTERM-containing proteins generally contain an N-terminal signal peptide and exhibit high diversity and little homology to known proteins. All bacteria with PEP-CTERM have both an outer membrane and exopolysaccharide (EPS) production genes. By a simple heuristic for screening phylogenetic profiles in the absence of pre-formed protein families, we discovered that a homolog of the membrane protein EpsH (exopolysaccharide locus protein H) occurs in a species when PEP-CTERM domains are found. The EpsH family contains invariant residues consistent with a transpeptidase function. Most PEP-CTERM proteins are encoded by single-gene operons preceded by large intergenic regions. In the Proteobacteria, most of these upstream regions share a DNA sequence, a probable cis-regulatory site that contains a sigma-54 binding motif. The phylogenetic profile for this DNA sequence exactly matches that of three proteins: a sigma-54-interacting response regulator (PrsR), a transmembrane histidine kinase (PrsK), and a TPR protein (PrsT). CONCLUSION: These findings are consistent with the hypothesis that PEP-CTERM and EpsH form a protein export sorting system, analogous to the LPXTG/sortase system of Gram-positive bacteria, and correlated to EPS expression. It occurs preferentially in bacteria from sediments, soils, and biofilms. The novel method that led to these findings, partial phylogenetic profiling, requires neither global sequence clustering nor arbitrary similarity cutoffs and appears to be a rapid, effective alternative to other profiling methods.

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MYXO-CTERM sorting tag directs proteins to the cell surface via the type II secretion system.

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