It has recently been proposed that PpiD is a periplasmic gatekeeper of the Sec translocon responsible for newly translocated OMPs [24]. Our work agrees with and refines this assumption, as it shows that PpiD exhibits find more the requisite chaperone activity for such a function, that this function is not preferentially directed at folding of OMPs, and that PpiD cooperates with SurA, Skp, FkpA and DegP in mediating protein folding in the periplasmic compartment of the cell. We suggest that the role of PpiD is to assist in the initial periplasmic folding events of many newly secreted envelope proteins. In the cytosol, the folding of newly synthesized proteins is initiated by the

ribosome-associated chaperone TF [45, 46]. Of note, PpiD

and TF show some interesting analogies. First, similar to PpiD TF is composed of three domains: an N-terminal ribosome-binding domain, a Dibutyryl-cAMP supplier central FKBP-like PPIase domain, and a C-terminal chaperone module which is structurally homologous to the chaperone module of SurA [41, 47] and, as outlined above, shows sequence similarity with the N-terminal putative chaperone region of PpiD. Second, TF associates with the ribosome to sequester and protect polyDuvelisib in vitro peptides just as they emerge from the peptide exit tunnel [46] and this association is crucial for its in vivo function [48]. PpiD on the other hand, is anchored OSBPL9 in the inner membrane and interacts with newly translocated polypeptides that emerge from the periplasmic exit site of the Sec translocon [24] and according to our data, the anchoring of PpiD in the membrane

is required for its function in vivo. Third, TF is dispensable for cell viability and a deletion of the tig gene confers a discernable phenotype only in combination with a mutation of the dnaK gene for the cytosolic chaperone DnaK [45]. Likewise, lack of PpiD gives a discernable phenotype only in cells with already compromised periplasmic chaperone activity, such as in the fkpA ppiD surA triple mutant and in the degP ppiD and ppiD skp double mutants. Finally, the amino acid sequence pattern of known PpiD binding peptides [44] resembles that of the peptide binding motifs identified for the cytosolic chaperones TF and DnaK, consisting of a central patch of hydrophobic amino acids flanked by positively charged amino acids [49]. Altogether, we speculate that PpiD may represent the periplasmic counterpart of TF. Its fixed localization in the inner membrane not necessarily conflicts with such a function, as it may provide a local enrichment of the binding partners but still allows PpiD to dynamically interact with and cycle on and off its interaction partners by lateral diffusion in the membrane, just as it is the hallmark of TF function on translating ribosomes [50].