Structure and function of protein translocation and localization system

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Protein translocation across the E.coli cytoplasmic membrane is catalyzed by a Sec macinery comprising six Sec factors. Among them, SecA undergoes the membrane-insertion and -deinsertion cycle upon the binding and hydrolysis of ATP, respectively. The SecA cycle is the direct driving force for protein translocation. SecG, a membrane component of the machinery, also undergoes a membrane topology inversion cycle, which is coupled to the SecA cycle. In this research project, we revealed that the SecG cycle facillitates the SecA cycle, thereby causing efficient protein translocation. Moreover, a protein motive force was also found to accerelate the SecA cycle.Maturation of lipoproteins takes place on the outer surface of the cytoplasmic membrane. Lipoproteins having other than Asp residue at the +2 position are released from the cytoplasmic membrane and localized to the outher membrane. We previously found that LolA, a lipoprotein-specific molecular chaperone in the periplasm, forms a complex with outer membrane-directed lipoprotein, but not inner membrane-specific one having Asp at +2 position. In this research project, we found that LolB and LolCDE complex are also essential for the sorting and membrane-specific localization of lipoproteins. LolB is an outer membrane receptor for lipoprotein. The LolCDE complex belongs to the ABC transport super family and mediates the release of lipoprotein from the cytoplasmic membrane.