Chaperone that functions as a gatekeeper on the periplasmic side of the SecYEG translocon. Facilitates the translocation of precursor proteins across SecYEG by interacting with the translocating substrate. Also plays a role in the release of newly synthesized secreted proteins at the periplasmic exit site of the Sec translocon. Interacts with the SecYEG translocon (By similarity). Binds to the lateral gate of SecY (By similarity). Forms a complex with YfgM (By similarity). Located at the lateral gate of SecY. Belongs to the PpiD chaperone family.