KdpD is a sensor kinase protein in the inner membrane of Escherichia colicontaining four transmembrane regions. The periplasmic loops connecting the transmembrane regions are intriguingly short and proteasemapping allowed us to only follow the translocation of the second periplasmic loop. The results show that neither the Sec translocase nor theYidCprotein are required formembrane insertion of the second loop ofKdpD.