EFFECTS OF NONLAMELLAR-PRONE LIPIDS ON THE ATPASE ACTIVITY OF SECA BOUND TO MODEL MEMBRANES

The effect of nonlamellar-prone lipids, diacylglycerol (DG) and phosph atidylethanolamine (PE), on the ATPase activity of SecA was examined. When Escherichia coli PE of the standard vesicles composed of 60 mol% of this lipid and 40 mol% of dioleoylphosphatidylglycerol (DOPG) is gr adually replaced with either dioleoylglycerol (DOG) or dioeloyl PE (DO PE), the ATPase activity of SecA present together increased appreciabl y. On the other hand, when E. coli PE of the standard vesicles was rep laced with DOG analogs, the SecA ATPase activity decreased slightly, a nd when replaced activity was more appreciable. When DOPE or E. coli P E was added to PC vesicles, the SecA ATPase activity was enhanced only slightly, suggesting that the hexagonal II structure per se is not im portant for the ATPase activity increase. It was observed that DOG ind uced phase separation of PG, and the lamellar-hexagonal II (L-H-II) 10 degrees C. The DOG analogs had no effect on these properties, suggest ing the importance of the phase separation of PG and the decrease of L -H-II transition temperature of lipid bilayers to the SecA ATPase acti vity. The phase separation of PG by Ca2+ also brought about increased ATPase activity of SecA, underlining the importance of phase separatio n of PG for the enzyme activity. The incorporation of DOG or DOPE in t he vesicle also increased the amount of SecA penetration into the memb rane. Studies with vesicles without SecA showed increased exposure of hydrophobic acyl chains when the DOG was present. Taken together, thes e observations suggest that the phase separation of PG and/or the bila yer penetration of SecA are mainly responsible for the enhanced SecA-v esicle interaction with concomitant increase in SecA ATPase activity.