Biochemical Analysis of Atlastin’s Membrane Anchor: Morphology, Dynamics, and Function
Betancourt, Miguel A
McNew, James A
Doctor of Philosophy
This project sheds new light into the endoplasmic reticulum (ER) fusion protein, atlastin. We studied its membrane anchor and its interaction with the lipid bilayer. The endoplasmic reticulum is composed flattened sheets and interconnected tubules that extend throughout the cytosol and contact other organelles. These discrete ER morphologies require specialized proteins that drive membrane curvature, dynamics, and mediate their maintenance. The GTPase atlastin is required for homotypic fusion of ER tubules. All atlastin homologs possess a conserved domain architecture consisting of a GTPase domain, a three-helix bundle middle domain, a hydrophobic membrane anchor, and a C-terminal tail. We analyzed atlastin’s hydrophobic anchor with recombinant atlastin and different mutants reconstituted into preformed liposomes, as model membranes. While traditionally atlastin’s membrane anchor was assumed to be two transmembrane segments that fully span the lipid bilayer; we have found it consists of two intramembranous hairpin loops. The topology of these hairpins remains static during membrane fusion and do not appear to play an active role in lipid mixing. We also analyzed the membrane domain topology of the mitochondrial fusion protein mitofusin-1 and ER resident protein Sac1 and found that they also have a dual intramembranous hairpin membrane anchor. This points to a conserved topology that may be expanded to ER resident protein and atlastin homologs. We were also able to recapitulate an ER-like network with atlastin proteoliposomes in polylysine coated coverslips; thus, showing that atlastin’s can form and maintain tubular structures, this result is consistent with the dual hairpin intramembrane loop topology. We also determined that co-reconstitution of atlastin with reticulon, an ER tube-forming protein, did not influence GTPase activity or membrane fusion, however both have the propensity to inhabit high curvature membranes. We also analyzed atlastin’s GTP binding pocket and found that inter- and intra-molecular salt bridging is important in GTP hydrolysis.