Fig. 3. Mechanisms of Arf GAP1 activation. (A) Effect of diacylglycerol on lipid packing. Diacylglycerol has a small head group compared with other lipids. When present in the membrane, lipid head groups pack less tightly, allowing peripheral membrane proteins, such as Arf GAP1, access to the central, hydrophobic portion of the bilayer. Arf GAP1 is more active in this environment. (B) Effect of membrane curvature on lipid packing and Arf GAP1 binding. On a tightly packed flat surface, Arf GAP1 cannot penetrate the bilayer and, therefore, is inactive. On the convex part of the bud, the packing of lipids is loosened allowing Arf GAP1 to penetrate the bilayer and consequently become active. GTP on Arf is therefore hydrolyzed and Arf-GDP dissociates from membrane.