Insights into the Mechanisms of Membrane Curvature and Vesicle Scission by the Small GTPase Sar1 in the Early Secretory Pathway

Abstract

The small GTPase protein Sar1 is known to be involved in both the initiation of COPII coated vesicle formation and scission of the nascent vesicle from the ER. The molecular details for the mechanism of membrane remodeling by Sar1 remain unresolved. Here we show that Sar1 transforms synthetic liposomes into structures of different morphologies including tubules and detached vesicles. We demonstrate that Sar1 alone is competent for vesicle scission in a manner that depends on the concentration of Sar1 molecules occupying the membrane. Sar1 molecules align on low curvature membranes to form an extended lattice. The continuity of this lattice breaks down as the curvature locally increases. The smallest repeating unit constituting the ordered lattice is a Sar1 dimer. The three dimensional structure of the Sar1 lattice was reconstructed by substituting spherical liposomes with galactoceramide lipid tubules of homogeneous diameter. These data suggest that Sar1 dimerization is responsible for the formation of constrictive membrane curvature. We propose a model whereby Sar1 dimers assemble into ordered arrays to promote membrane constriction and COPII-directed vesicle scission.

Computational and biochemical characterization of two partially overlapping interfaces and multiple weak-affinity K-Ras dimers

“…Some studies supported direct interaction involving the catalytic domain and proposed several possible interfaces913. Direct protein-protein interaction has also been found in other small GTPases such as Arf15, Rab16 and Sar117. However, dimerization primarily mediated by the polybasic C-terminal membrane anchor has been reported for Rho18 and Rsr119.…”

Cargo adaptors: structures illuminate mechanisms regulating vesicle biogenesis

“…Accordingly, exomer was found to cooperate with Arf1 to drive membrane fission in vitro, and the exomer membrane insertion element was important for this activity both in vitro and in vivo . Arf1 (and the related Sar1 GTPase) can remodel membranes via an N-terminal amphipathic helix [93, 105–108] , and exomer appears to amplify this intrinsic capability of Arf1. The ability of exomer to bind and orient two Arf1 molecules, on both flat and highly curved membrane surfaces (such as a Golgi membrane and at the neck of a budding vesicle), likely arises from the hinge-motion afforded by the Chs5 N-terminal domain [86, 88] (Figure 4).…”

Membrane Transport at an Organelle Interface in the Early Secretory Pathway: Take Your Coat Off and Stay a While

“…The first is based on the premise that Sar1 action is restricted to the ER, where it promotes membrane tubulation and ultimately accumulates at the neck of nascent COPII‐coated buds to facilitate bilayer scission upon GTP hydrolysis. This idea is supported by studies demonstrating the ability of recombinant Sar1 to create membrane tubules (Figure ), as well as more recent work demonstrating that Sar1 is capable of sensing membrane curvature . Additionally, the intrinsic GTPase activity of Sar1 is stimulated in the presence of elevated curvature, suggesting a mechanism by which it may drive transport carrier scission .…”