title: Sar1 GTPase Activity Is Regulated by Membrane Curvature.
abstract: The majority of biosynthetic secretory proteins initiate their journey through the endomembrane system from specific subdomains of the endoplasmic reticulum. At these locations, coated transport carriers are generated, with the Sar1 GTPase playing a critical role in membrane bending, recruitment of coat components, and nascent vesicle formation. How these events are appropriately coordinated remains poorly understood. Here, we demonstrate that Sar1 acts as the curvature-sensing component of the COPII coat complex and highlight the ability of Sar1 to bind more avidly to membranes of high curvature. Additionally, using an atomic force microscopy-based approach, we further show that the intrinsic GTPase activity of Sar1 is necessary for remodeling lipid bilayers. Consistent with this idea, Sar1-mediated membrane remodeling is dramatically accelerated in the presence of its guanine nucleotide-activating protein (GAP), Sec23-Sec24, and blocked upon addition of guanosine-5'-[(β,γ)-imido]triphosphate, a poorly hydrolysable analog of GTP. Our results also indicate that Sar1 GTPase activity is stimulated by membranes that exhibit elevated curvature, potentially enabling Sar1 membrane scission activity to be spatially restricted to highly bent membranes that are characteristic of a bud neck. 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.
authors.affiliation: From the Department of Biomolecular Chemistry, University of Wisconsin-Madison School of Medicine and Public Health, Madison, Wisconsin 53706.
authors.forename: Michael G
authors.initials: MG
authors.lastname: Hanna
authors.affiliation: the Department of Pharmacology, University of Cambridge, Tennis Court Road, CB2 1PD Cambridge, United Kingdom, and.
authors.forename: Ioanna
authors.initials: I
authors.lastname: Mela
authors.affiliation: From the Department of Biomolecular Chemistry, University of Wisconsin-Madison School of Medicine and Public Health, Madison, Wisconsin 53706.
authors.forename: Lei
authors.initials: L
authors.lastname: Wang
authors.affiliation: the Department of Pharmacology, University of Cambridge, Tennis Court Road, CB2 1PD Cambridge, United Kingdom, and.
authors.forename: Robert M
authors.initials: RM
authors.lastname: Henderson
authors.affiliation: the Department of Neuroscience, University of Wisconsin-Madison School of Medicine and Public Health, Madison, Wisconsin 53705.
authors.forename: Edwin R
authors.initials: ER
authors.lastname: Chapman
authors.affiliation: the Department of Pharmacology, University of Cambridge, Tennis Court Road, CB2 1PD Cambridge, United Kingdom, and.
authors.forename: J Michael
authors.initials: JM
authors.lastname: Edwardson
authors.affiliation: From the Department of Biomolecular Chemistry, University of Wisconsin-Madison School of Medicine and Public Health, Madison, Wisconsin 53706, audhya@wisc.edu.
authors.forename: Anjon
authors.initials: A
authors.lastname: Audhya
mesh_terms: D019943:Amino Acid Substitution; D000818:Animals; D022181:COP-Coated Vesicles / Q000187:drug effects / Q000378:metabolism* / Q000648:ultrastructure; D017173:Caenorhabditis elegans / Q000187:drug effects / Q000201:enzymology / Q000502:physiology* / Q000648:ultrastructure; D029742:Caenorhabditis elegans Proteins / Q000037:antagonists & inhibitors / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D004721:Endoplasmic Reticulum / Q000378:metabolism* / Q000648:ultrastructure; D004791:Enzyme Inhibitors / Q000494:pharmacology; D020558:GTP Phosphohydrolases / Q000037:antagonists & inhibitors / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D020690:GTPase-Activating Proteins / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism; D006160:Guanosine Triphosphate / Q000378:metabolism*; D006165:Guanylyl Imidodiphosphate / Q000494:pharmacology; D006801:Humans; D008051:Lipid Bilayers / Q000737:chemistry / Q000378:metabolism; D021962:Membrane Microdomains / Q000187:drug effects / Q000378:metabolism / Q000648:ultrastructure; D018625:Microscopy, Atomic Force; D008954:Models, Biological; D020559:Monomeric GTP-Binding Proteins / Q000037:antagonists & inhibitors / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D009154:Mutation; D053140:Organelle Shape / Q000187:drug effects; D034622:RNA Interference; D020794:Receptor Protein-Tyrosine Kinases / Q000235:genetics / Q000378:metabolism; D011994:Recombinant Proteins / Q000737:chemistry / Q000378:metabolism; D033921:Vesicular Transport Proteins / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism; D020703:ras GTPase-Activating Proteins / Q000235:genetics / Q000378:metabolism
chemical_list: D029742:Caenorhabditis elegans Proteins; D004791:Enzyme Inhibitors; C108739:GAP-1 protein, C elegans; D020690:GTPase-Activating Proteins; D008051:Lipid Bilayers; D011994:Recombinant Proteins; C000607941:SEC-24 protein, C elegans; D033921:Vesicular Transport Proteins; D020703:ras GTPase-Activating Proteins; C512737:sec-23 protein, C elegans; D006165:Guanylyl Imidodiphosphate; D006160:Guanosine Triphosphate; D020794:Receptor Protein-Tyrosine Kinases; D020558:GTP Phosphohydrolases; C000607940:SAR-1 protein, C elegans; C513282:SAR1B protein, human; D020559:Monomeric GTP-Binding Proteins
keywords: COPII; GTPase; endoplasmic reticulum (ER); membrane bilayer; membrane transport