Chaperone that functions as a gatekeeper on the periplasmic side of the SecYEG translocon (PubMed:18439025, PubMed:20920237, PubMed:29097228). Facilitates the translocation of precursor proteins across SecYEG by interacting with the translocating substrate (PubMed:29097228). Also plays a role in the release of newly synthesized secreted proteins at the periplasmic exit site of the Sec translocon (PubMed:18439025, PubMed:20920237, PubMed:29097228). May be involved in the early periplasmic folding of many newly translocated proteins (PubMed:18439025, PubMed:20920237). Acts as a transient subunit of the Sec translocon that binds to the lateral gate of SecY and is detached by nascent membrane proteins but not by SecA (PubMed:24951590). Does not have peptidyl-prolyl isomerase (PPIase) activity. PPIase activity could not be generated by substitutions at the peptide binding site of the isolated parvulin-like domain. (Microbial infection) May play an important role in bacteriophage T4 infection and be involved in the penetration of the inner membrane by the bacteriophage injection machinery, resulting in a DNA-conducting channel to translocate the phage DNA into the interior of the cell. Interacts with the SecYEG translocon (PubMed:24951590, PubMed:29097228, PubMed:31699901). Binds to the lateral gate of SecY (PubMed:24951590, PubMed:31699901). Forms a complex with YfgM (PubMed:21210718, PubMed:24855643, PubMed:31699901). Also interacts with YidC (PubMed:31699901). Interacts with peptide substrates and misfolded proteins (PubMed:18498364, PubMed:19866485). Has been isolated as a homodimer and homotrimer from inner membrane preparations (PubMed:16079137). (Microbial infection) Interacts with the central spike complex of bacteriophage T4. Located at the lateral gate of SecY. By heat shock via the sigma factor RpoH (PubMed:9670013). Member of the two-component system CpxA/CpxR regulon (PubMed:9670013). Consists of an N-terminal helix that anchors PpiD in the inner membrane, followed by three domains that face the periplasm. The first or the third domain are probably chaperone domains. The second domain is a parvulin-like domain, which is devoid of catalytic activity. It shows a parvulin fold and resembles most closely the inactive first parvulin domain of SurA, which is part of the chaperone unit of this protein and presumably involved in substrate recognition. Deletion of the gene retards the release of a translocating outer membrane protein into the periplasm (PubMed:18439025). Null mutation leads to the induction of the periplasmic stress response. Null mutants are hypersensitive to hydrophobic antibiotics such as novobiocin and to detergents such as SDS, and exhibit altered outer membrane proteins profile (PubMed:9670013). In contrast, another study shows that inactivation of the gene has no discernible effect on the levels of outer membrane proteins (PubMed:16267292). Lack of the gene confers increased temperature-sensitivity in a degP mutant (PubMed:20920237). (Microbial infection) Deletion of the gene leads to partial reduction in the plating efficiency of bacteriophage T4. Overexpression restores viability of surA skp double deletion mutant but it does not completely compensate for the growth defect caused by the simultaneous lack of the SurA and Skp chaperones. Suppression of surA skp lethality does not require the parvulin domain but the membrane-localization of PpiD. In the absence of both SurA and Skp, overproduction of PpiD can, at least in part, counteract the defects in the biogenesis of OmpA and possibly of other OMPs. Belongs to the PpiD chaperone family. Was originally thought to be a peptidyl-prolyl isomerase (PPIase). It was shown later that even if PpiD shows sequence similarity to PPIases, it is in fact devoid of PPIase activity (PubMed:19866485).