From eukaryotes to prokaryotes, proper protein folding is essential to cellular function. Disulfide bond formation contributes to the overall protein folding process, stabilizing structures and protecting against degradation. Disulfide bond-forming machines that facilitate proper protein folding are well recognized in eukaryotes and Gram-negative bacteria. In contrast, a major disulfide bond-forming pathway has only recently been identified in the Gram-positive Actinobacteria Actinomyces oris, Corynebacterium diphtheriae, and Corynebacterium matruchotii. In these organisms, a membrane-bound thiol-disulfide oxidoreductase named MdbA catalyzes post- translocational folding of exported proteins. Importantly, genetic disruption of mdbA abrogates assembly of adhesive pili and biofilm formation, alters cell morphology, and attenuates bacterial virulence. Nonetheless, how actinobacterial cells cope with stress and protein misfolding is not well understood. To address this fundamental question, we began to analyze the proteomes of Actinobacteria and found that most PBPs harbor 2 or more cysteines; intriguingly, deletion of pbp1A or pbp1B in C. diphtheriae resulted in a cell morphology defect that mirrors that of mdbA mutations. With a genetic approach, we then screened for viable suppressor mutants when C. diphtheriae mdbA mutant cells grown at non-permissive temperatures. Serendipitously, we discovered another thiol-disulfide oxidoreductase, which we named TsdA (tsd for temperature-sensitive dsb-forming). Preliminary studies reveal that TsdA contains a thioredoxin-like fold found in MdbA, suggesting that TsdA may serve as a specialized disulfide bond-forming machine to encounter cell stress. Finally, we identified a potential protein disulfide bond isomerase that may serve as a safeguarding system to rescue misfolded proteins. As we continue employing A. oris and C. diphtheriae as experimental models in this renewal application, by using a multidisciplinary approach that combines genetics, biochemical and biofilm assays, and crystallography, we aim to examine the molecular coupling between oxidative protein folding and cell wall biosynthesis in Actinobacteria, to elucidate the mechanism of oxidative protein folding mediated by a compensatory thiol-oxidoreductase machine in response to stress, and to elucidate a pathway for protein disulfide bond isomerization in Actinobacteria. .
This study examines how oral Actinobacteria maintain proper folding of virulence factors during their assembly and under stress conditions. A clear understanding of these aspects will provide new targets for anti-infective therapies and prevention of oral biofilm-associated diseases.
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