Processing of preproteins by liposomes bearing leader peptidase

Procoat, the precursor form of M13 coat protein, assembles into sealed liposomes bearing only internally oriented leader peptidase and is processed to yield transmembrane coat protein [Ohno-Iwashita, Y., & Wickner, W. (1983) J. Biol. Chem. 258, 1895-1900]. The precursors of maltose-binding protein and of outer membrane protein A (OmpA) are also processed by these liposomes, showing that these preproteins can at least partially insert across a lipid bilayer. The ability to insert into a bilayer may be a general property of preproteins. The cleavage products, mature OmpA and maltose-binding protein, are not sequestered within the liposomes, suggesting that an additional factor(s) is (are) required for complete translocation. Liposomes were also prepared with leader peptidase in a more physiological, membrane-spanning orientation. These liposomes were also active in the cleavage of externally added procoat, pro-OmpA, and pre maltose-binding protein, though the mature OmpA and maltose-binding protein were still not sequestered within the liposomes. Pretreatment of these liposomes with trypsin cleaved near the amino terminus of the leader peptidase, inactivating the enzyme. The function of this amino-terminal domain, on the opposite side of the membrane from the catalytic domain, is unknown.