Abstract
Clostridioides difficile is the leading cause of antibiotic-associated infections worldwide. Within the host, C. difficile can transition from a sessile to a motile state by secreting PPEP-1, which releases the cells from the intestinal epithelium by cleaving adhesion proteins. PPEP-1 belongs to the group of Pro-Pro endopeptidases (PPEPs), which are characterized by their unique ability to cleave proline-proline bonds. Interestingly, another putative member of this group, CD1597, is present in C. difficile. Although it possesses a domain similar to other PPEPs, CD1597 displays several distinct features that suggest a markedly different role for this protein. We investigated the proteolytic activity of CD1597 by testing various potential substrates. In addition, we investigated the effect of the absence of CD1597 by generating an insertional mutant of the cd1597 gene. Using the cd1597 mutant, we sought to identify phenotypic changes through a series of in vitro experiments and quantitative proteomic analyses. Furthermore, we aimed to study the localization of this protein using a fluorogenic fusion protein. Despite its similarities to PPEP-1, CD1597 did not show proteolytic activity. In addition, the absence of CD1597 caused an increase in various sporulation proteins during the stationary phase, yet we did not observe any alterations in the sporulation frequency of the cd1597 mutant. Furthermore, a promoter activity assay indicated a very low expression level of cd1597 in vegetative cells, which was independent of the culture medium and growth stage. The low expression was corroborated by our comprehensive proteomic analysis of the whole cell cultures, which failed to identify CD1597. However, an analysis of purified C. difficile spores identified CD1597 as part of the spore proteome. Hence, we predict that the protein is involved in sporulation, although we were unable to define a precise role for CD1597 in C. difficile.