Mitochondrial Membrane Fusion: Computational Modeling of Mitofusins

Abstract

Fzo1, a large GTPase of the Dynamin-Related Proteins superfamily, is a key component in mitochondrial outer membrane fusion and is required for maintaining mitochondrial dynamics and morphology. The protein is anchored to the outer membrane by two transmembrane segments and its N-terminal GTPase domain and C-terminal are exposed to the cytosol. Recent data indicate that the GTPase domain of Fzo1 would induce a conformational change concomitant with mitochondrial tethering, thus promoting membrane fusion [1]. We investigate the structure and dynamics of Fzo1 through molecular modeling and all-atom simulation in a model mixed lipid bilayer, closely linked to experiments. Our structural model integrates information from several template structures, experimental knowledge, as well as ab initio models of the transmembrane segments that are unique to Fzo1. The model is validated experimentally through charge swap mutations across predicted salt bridges and a series of N-terminal truncation mutants indicates that this region is dispensable for function. Our approach unravels hinges domains involved in the conformational change and identified critical residues required for protein stability. Moreover several point mutation found to disrupt the architecture of the protein are located in the coiled-coil domain which has been shown fundamental for the protein [2]. Finally, we dissected key residues in protein-GDP interaction providing fundamental insights about molecular mechanisms by which mitofusins catalyze membrane fusion.[1] Cohen, M.M., Amiott, E.A., Day, A.R., Leboucher, G.P., Erin, N.P., Glickman, M.H., McCaffery, J.M, Shaw, J.M. and Weissman, A.M. (2011). J. Cell Sci. 124, 1403-1410.[2] Griffin, E. E. and Chan, D. C. (2006). J. Biol. Chem. 281, 16599-16606.