A Neisserial autotransporter NalP modulating the processing of other autotransporters

Autotransporters constitute a relatively simple secretion system in Gram-negative bacteria, depending for their translocation across the outer membrane only on a C-terminal translocator domain. We have studied a novel autotransporter serine protease, designated NalP, from Neisseria meningitidis strain H44/76, featuring a lipoprotein motif at the signal sequence cleavage site. Indeed, lipidation of NalP could be demonstrated, but the secreted 70 kDa domain of NalP lacked the lipid-moiety as a result of additional N-terminal processing. A nalP mutant showed a drastically altered profile of secreted proteins. Mass-spectrometric analysis of tryptic fragments identified the autotransporters IgA protease and App, a homologue of the adhesin Hap of Haemophilus influenzae, as the major secreted proteins. Two forms of both of these proteins were found in the culture supernatant of the wild-type strain, whereas only the lower molecular-weight forms predominated in the culture supernatant of the nalP mutant. The serine-protease active site of NalP was required for the modulation of the processing of these autotransporters. We propose that, apart from the autoproteolytic processing, NalP can process App and IgA protease and hypothesize that this function of NalP could contribute to the virulence of the organism.