Membrane proteins need to be properly inserted and folded in the membrane in order to perform a range of activities that are essential for the survival of bacteria. The Sec translocon and the YidC insertase are responsible for the insertion of the majority of proteins into the cytoplasmic membrane. YidC can act in combination with the Sec translocon in the insertion and folding of membrane proteins. However, YidC also functions as an insertase independently of the Sec translocon for so-called YidC-only substrates. In addition, YidC can act as a foldase and promote the proper assembly of membrane protein complexes. Here, we investigate the effect of Escherichia coli YidC depletion on the assembly of Penicillin Binding Proteins (PBPs), that are involved in cell wall synthesis. YidC depletion does not affect the total amount of the specific cell division PBP3 (FtsI) in the membrane, but the amount of active PBP3, as assessed by substrate binding, is reduced two-fold. A similar reduction in the amount of active PBP2 was observed, while the levels of active PBP1A/1B and PBP5 were essentially similar. PBP1B and PBP3 disappeared from higher Mw bands upon YidC depletion, indicating that YidC might play a role in PBP complex formation. Taken together, our results suggest that the foldase activity of YidC can extend to the periplasmic domains of membrane proteins.
IMPORTANCE: This work addresses the role of the membrane protein insertase YidC in the biogenesis of Penicillin Binding Proteins (PBPs). PBPs are proteins containing one transmembrane segment and a large periplasmic or extracellular domain, which are involved in peptidoglycan synthesis. We observe that in the absence of YidC, two critical PBPs are not correctly folded even though the total amount of protein in the membrane is not affected. Our findings extend the function of YidC as a foldase for membrane protein (complexes) to periplasmic domains of membrane proteins.