The chlamydial
protease/
proteasome-like activity factor (
CPAF) is secreted into the host cytosol to degrade various host factors that benefit chlamydial intracellular survival. Although the full-length
CPAF is predicted to contain a putative
signal peptide at its N terminus, the secretion pathway of
CPAF is still unknown. Here, we have provided experimental evidence that the N-terminal sequence covering the M1-G31 region was cleaved from
CPAF during chlamydial
infection. The
CPAF N-terminal sequence, when expressed in a phoA gene fusion construct, was able to direct the export of the mature PhoA
protein across the inner membrane of wild-type Escherichia coli. However, E. coli mutants deficient in SecB failed to support the
CPAF signal-peptide-directed secretion of PhoA. Since native PhoA secretion was known to be independent of SecB, this SecB dependence must be rendered by the
CPAF leader peptide. Furthermore, lack of SecY function also blocked the
CPAF signal-peptide-directed secretion of PhoA. Most importantly,
CPAF secretion into the host cell cytosol during chlamydial
infection was selectively inhibited by an inhibitor specifically targeting
type I signal peptidase but not by a
type III secretion-system-specific inhibitor. Together, these observations have demonstrated that the chlamydial
virulence factor CPAF relies on Sec-dependent transport for crossing the chlamydial inner membrane, which has provided essential information for further delineating the pathways of
CPAF action and understanding chlamydial pathogenic mechanisms.