GTP Hydrolysis on Sar1 Is Regulated by Membrane Curvature and Facilitates Vesicle Formation♦ Papers of the Week

♦ See referenced article, J. Biol. Chem. 2016, 291, 1014–1027Many proteins travel through the secretory pathway in lipid-bound transport carriers. Most of the transport carriers are generated at specific locations on the endoplasmic reticulum where the cytosolic coat protein II (COPII) complex plays a role. A GTPase called Sar1 is a member of the COPII complex. It is involved in membrane bending, recruitment of other COPII coat proteins, and vesicle formation. However, the coordination of these different activities by Sar1 is not well understood. In this Paper of the Week, Anjon Audhya at the University of Wisconsin-Madison School of Medicine and Public Health and colleagues showed that Sar1 senses membrane curvature to bind more avidly to highly curved membranes. The investigators also showed that the GTPase activity of Sar1 increases when the membranes are more curved. The authors comment, "Taken together, our data support a stepwise model in which the amino-terminal amphipathic helix of GTP-bound Sar1 stably penetrates the endoplasmic reticulum membrane, promoting local membrane deformation. As membrane bending increases, Sar1 membrane binding is elevated, ultimately culminating in GTP hydrolysis, which may destabilize the bilayer sufficiently to facilitate membrane fission."jbc;291/3/1028/FU1F1FU1Representative atomic force microscopy images of supported lipid bilayers composed of 70% phosphatidylcholine, 15% phosphatidylethanolamine, and 15% phosphatidylserine imaged over time, following assembly in the presence of GDP-bound Sar1 (400 nm) and supplementation with a buffer containing 1 mm GTP. The times shown are relative to the timing of GTP addition. Scale bar, 250 nm.

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