Left, model for the Mla lipid transport system. MlaC likely serves as a soluble lipid carrier protein to transport lipids between the OM MlaA-OmpC/OmpF complex and the IM MlaFEDB complex. Middle left, PqiB may interact with OM lipoprotein PqiC and IM protein PqiA to mediate periplasmic lipid transport. Middle right, YebT may associate with YebS in the IM as part of a larger transport complex. Right, model of LPS transport system based on other studies (see references in text), which has also been suggested to form a bridge for LPS transport across the periplasm using a structurally unrelated framework.