Characterisation of preYvaY export reveals differences in the substrate specificities of Bacillus subtilis and Escherichia coli leader peptidases.

Translocation, processing and secretion of YvaY, a Bacillus subtilis protein of unknown function, were characterised both in B. subtilis and in Escherichia coli. In its natural host B. subtilis, YvaY was transiently synthesised at the end of the exponential growth phase. It was efficiently secreted into the culture supernatant in spite of a calculated membrane spanning domain in the mature part of the protein. In E. coli, despite the high conservation of Sec-dependent transport components, processing of preYvaY was strongly impaired. To uncover which elements of E. coli and B. subtilis translocation systems are responsible for the observed substrate specificity, components of the B. subtilis Sec-system were co-expressed besides yvaY in E. coli. Expression of B. subtilis secA or secYEG genes did not affect processing, but expression of B. subtilis signal peptidase genes significantly enhanced processing of preYvaY in E. coli. While the major signal peptidases SipS or SipT had a strong stimulatory effect on preYvaY processing, the minor signal peptidases SipU, SipV or SipW had a far less stimulatory effect in E. coli. These results reveal that targeting and translocation of preYvaY is mediated by the E. coli Sec proteins but processing of preYvaY is not performed by E. coli signal peptidase LepB. Thus, differences in substrate specificities of E. coli LepB and the B. subtilis Sip proteins provide the bottleneck for export of YvaY in E. coli. Significant slower processing of preYvaY in absence of SecB indicated that SecB mediates targeting of the B. subtilis precursor.