Membrane localization and topology of the Yersinia pestis YscJ lipoprotein

The localization and membrane topology of the Yersinia pestis YscJ lipoprotein, an essential component of the type III secretion apparatus, was investigated. YscJ was demonstrated to be an inner membrane (IM) lipoprotein that is anchored to the periplasmic face of the IM via an N-terminal lipid moiety and via a C-terminal transmembrane (TM) domain. Localization of the N-terminal lipid moiety to the IM occurred regardless of the amino-acid residues found in the +2 or +3 positions. IM localization was dependent upon an intact N-terminal domain (amino acids +1 to +61), suggesting that this region plays a role in YscJ localization. In contrast, the YscJ C-terminal domain and TM domain were not required for IM localization. N-terminal sequence analysis demonstrated that a significant proportion of membrane-localized YscJ lacks N-acylation, the final modification required for Lol-dependent transport of a lipoprotein to the OM. Interestingly, attachment of the N-terminus to the IM was required for YscJ function; however, the YscJ secretion signal and lipo-box could be functionally replaced by the first TM domain of the YscV protein, suggesting that the mechanism of attachment to the IM was not critical.