Neisseria gonorrhoeae PilA is an FtsY homolog

The piLA gene of Neisseria gonorrhoeae was initially identified in a screen for transcriptional regulators of pilE, the expression locus for pilin, th e major structural component of gonococcal pill, The predicted protein sequ ence for PilA has significant homology to two GTPases of the mammalian sign al recognition particle (SRP), SRP54 and SR alpha. Homologs of SRP54 and SR alpha were subsequently identified in bacteria (Ffh and FtsY, respectively ) and appear to form an SRP-like apparatus in prokaryotes, Of the two prote ins, PilA is the most similar to FtsY (37% identical and 67% similar at the amino acid level), Like FtsY, PilA is essential for viability and hydrolyz es GTP. The similarities between PilA and the bacterial FtsY led us to ask whether PilA might function as the gonococcal FtsY, In this work, we show t hat overproduction of PilA in Escherichia coli leads to an accumulation of pre-P-lactamase, similar to previous observations with other bacterial SRP components. Low-level expression of pilA in an ftsY conditional mutant can complement the ftsY mutation and restore normal growth to this strain under nonpermissive conditions, In addition, purified PilA can replace FtsY in a n in vitro translocation assay using purified E. coli SRP components. A Pil A mutant that is severely affected in its GTPase activity cannot replace Ft sY in vivo or in vitro, However, overexpression of the GTPase mutant leads to the accumulation of pre-beta-lactamase, suggesting that the mutant prote in may interact with the SRP apparatus to affect protein maturation. Taken together, these results show that the gonococcal PilA is an FtsY homolog an d that the GTPase activity is necessary for its function.