Vectorial transport and folding of an autotransporter virulence protein during outer membrane secretion

Analysis of a beta-helical region in the p55 domain of Helicobacter pylori vacuolating toxin

Background

Helicobacter pylori is a gram-negative bacterium that colonizes the human stomach and contributes to the development of gastric cancer and peptic ulcer disease. VacA, a toxin secreted by H. pylori, is comprised of two domains, designated p33 and p55. Analysis of the crystal structure of the p55 domain indicated that its structure is predominantly a right-handed parallel β-helix, which is a characteristic of autotransporter passenger domains. Substitution mutations of specific amino acids within the p33 domain abrogate VacA activity, but thus far, it has been difficult to identify small inactivating mutations within the p55 domain. Therefore, we hypothesized that large portions of the p55 domain might be non-essential for vacuolating toxin activity. To test this hypothesis, we introduced eight deletion mutations (each corresponding to a single coil within a β-helical segment spanning VacA amino acids 433-628) into the H. pylori chromosomal vacA gene.

Results

All eight of the mutant VacA proteins were expressed by the corresponding H. pylori mutant strains and underwent proteolytic processing to yield ~85 kDa passenger domains. Three mutant proteins (VacA Δ484-504, Δ511-536, and Δ517-544) were secreted and induced vacuolation of mammalian cells, which indicated that these β-helical coils were dispensable for vacuolating toxin activity. One mutant protein (VacA Δ433-461) exhibited reduced vacuolating toxin activity compared to wild-type VacA. Other mutant proteins, including those containing deletions near the carboxy-terminal end of the β-helical region (amino acids Val⁵⁵⁹-Asn⁶²⁸), exhibited marked defects in secretion and increased susceptibility to proteolytic cleavage by trypsin, which suggested that these proteins were misfolded.

Conclusions

These results indicate that within the β-helical segment of the VacA p55 domain, there are regions of plasticity that tolerate alterations without detrimental effects on protein secretion or activity, as well as a carboxy-terminal region in which similar alterations result in protein misfolding and impaired secretion. We propose that non-essential β-helical coils and a carboxy-terminal β-helical segment required for proper protein folding and secretion are features shared by numerous autotransporter passenger domains.

The Bam (Omp85) complex is involved in secretion of the autotransporter haemoglobin protease

Autotransporters are large virulence factors secreted by Gram-negative bacteria. They are synthesized with a C-terminal domain that forms a β-barrel pore in the outer membrane implicated in translocation of the upstream ‘passenger’ domain across the outer membrane. However, recent structural data suggest that the diameter of the β-barrel pore is not sufficient to allow the passage of partly folded structures observed for several autotransporters. Here, we have used a stalled translocation intermediate of the autotransporter Hbp to identify components involved in insertion and translocation of the protein across the outer membrane. At this intermediate stage the β-domain was not inserted and folded as an integral β-barrel in the outer membrane whereas part of the passenger was surface exposed. The intermediate was copurified with the periplasmic chaperone SurA and subunits of the Bam (Omp85) complex that catalyse the insertion and assembly of outer-membrane proteins. The data suggest a critical role for this general machinery in the translocation of autotransporters across the outer membrane.

Measuring cotranslational folding of nascent polypeptide chains on ribosomes

Protein folding has been studied extensively in vitro, but much less is known about how folding proceeds in vivo. A particular distinction of folding in vivo is that folding begins while the nascent polypeptide chain is still undergoing synthesis by the ribosome. Studies of cotranslational protein folding are inherently much more complex than classical in vitro protein folding studies, and historically there have been few methods available to produce the quantities of pure material required for biophysical studies of the nascent chain, or assays to specifically interrogate its conformation. However, the past few years have produced dramatic methodological advances, which now place cotranslational folding studies within reach of more biochemists, enabling a detailed comparison of the earliest stages of protein folding on the ribosome to the wealth of information available for the refolding of full-length polypeptide chains in vitro.

Interaction of an autotransporter passenger domain with BamA during its translocation across the bacterial outer membrane

Autotransporters are a superfamily of virulence factors produced by Gram-negative bacteria consisting of a large N-terminal extracellular domain ("passenger domain") and a C-terminal beta barrel domain ("beta domain"). The mechanism by which the passenger domain is translocated across the outer membrane (OM) is unknown. Here we show that the insertion of a small linker into the passenger domain of the Escherichia coli O157:H7 autotransporter EspP effectively creates a translocation intermediate by transiently stalling translocation near the site of the insertion. Using a site-specific photocrosslinking approach, we found that residues adjacent to the stall point interact with BamA, a component of a heterooligomeric complex (Bam complex) that catalyzes OM protein assembly, and that residues closer to the EspP N terminus interact with the periplasmic chaperones SurA and Skp. The EspP-BamA interaction was short-lived and could be detected only when passenger domain translocation was stalled. These results support a model in which molecular chaperones prevent misfolding of the passenger domain before its secretion and the Bam complex catalyzes both the integration of the beta domain into the OM and the translocation of the passenger domain across the OM in a C- to N-terminal direction.