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

• Published in: Journal of molecular biology Vol. 426; no. 22; pp. 3811 - 3826
• Main Authors: Hariri, Hanaa, Bhattacharya, Nilakshee, Johnson, Kerri, Noble, Alex J., Stagg, Scott M.
• Format: Journal Article
• Language: English
• Published: England Elsevier Ltd 11.11.2014
• Subjects: ADE model; Cell Membrane - chemistry; Cell Membrane - metabolism; COP-Coated Vesicles - metabolism; dimerization; Endoplasmic Reticulum - metabolism; Guanosine Triphosphate - metabolism; Humans; Hydrolysis; Liposomes; membrane scission; Monomeric GTP-Binding Proteins - genetics; Monomeric GTP-Binding Proteins - metabolism; Mutation - genetics; N-terminal helix; Protein Multimerization; Sar1 lattice; Secretory Pathway - physiology; ADE; cryoEM; ADE model; EM; dimerization; GUV; DLS; ER; Sar1 lattice; 3D; 2D; membrane scission; CTF; GAP; N-terminal helix; cryoET; TEM
• ISSN: 0022-2836; 1089-8638
• DOI: 10.1016/j.jmb.2014.08.023

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 endoplasmic reticulum. 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. [Display omitted] •Vesicle scission in vitro depends on the concentration of Sar1 at the surface of the membrane.•Membrane deformation depends on the nucleotide state of Sar1.•Sar1 forms an ordered lattice of Sar1 dimers.•Assembly of the Sar1 lattice depends, in part, on the degree of surface curvature.