A Type I Signal Peptidase Is Required for Pilus Assembly in the Gram-Positive, Biofilm-Forming Bacterium Actinomyces oris

The Gram-positive bacteriumActinomyces oris , a key colonizer in the development of oral biofilms, contains 18 LPXTG motif-containing proteins, including fimbrillins that constitute two fimbrial types critical for adherence, biofilm formation, and polymicrobial interactions. Export of these protein precursors, which harbor a signal peptide, is thought to be mediated by the Sec machine and require cleavage of the signal peptide by type I signal peptidases (SPases). Like many Gram-positive bacteria,A. oris expresses two SPases, named LepB1 and LepB2. The latter has been linked to suppression of lethal “glyco-stress,” caused by membrane accumulation of the LPXTG motif-containing glycoprotein GspA when the housekeeping sortasesrtA is genetically disrupted. Consistent with this finding, we show here that a mutant lackinglepB2 andsrtA was unable to produce high levels of glycosylated GspA and hence was viable. However, deletion of neitherlepB1 norlepB2 abrogated the signal peptide cleavage and glycosylation of GspA, indicating redundancy of SPases for GspA. In contrast, thelepB2 deletion mutant failed to assemble the wild-type levels of type 1 and 2 fimbriae, which are built by the shaft fimbrillins FimP and FimA, respectively; this phenotype was attributed to aberrant cleavage of the fimbrillin signal peptides. Furthermore, thelepB2 mutants, including the catalytically inactive S101A and K169A variants, exhibited significant defects in polymicrobial interactions and biofilm formation. Conversely,lepB1 was dispensable for the aforementioned processes. These results support the idea that LepB2 is specifically utilized for processing of fimbrial proteins, thus providing an experimental model with which to study the basis of type I SPase specificity.IMPORTANCE Sec-mediated translocation of bacterial protein precursors across the cytoplasmic membrane involves cleavage of their signal peptide by a signal peptidase (SPase). Like many Gram-positive bacteria,A. oris expresses two SPases, LepB1 and LepB2. The latter is a genetic suppressor of lethal “glyco-stress” caused by membrane accumulation of glycosylated GspA when the housekeeping sortasesrtA is genetically disrupted. We show here that LepB1 and LepB2 are capable of processing GspA, whereas only LepB2 is required for cleavage of fimbrial signal peptides. This is the first example of a type I SPase dedicated to LPXTG motif-containing fimbrial proteins. Thus,A. oris provides an experimental model with which to investigate the specificity mechanism of type I SPases.