Mechanism of mitochondrial dynamin adaptor binding and membrane recruitment

Many cellular membrane fission events are mediated by dynamin-related proteins (DRP), which self-assemble into spirals encircling the lipid tubes at their sites of action. DRPs are GTPases whose activities are dramatically stimulated upon assembly. Rapid GTP hydrolysis causes constriction of DRP spirals and leads to membrane fission. DRPmediated membrane fission is initiated by the recruitment of a certain DRP to the specific cellular membrane via adaptor proteins. The adaptors also co-assemble with DRPs to form active fission complexes. The molecular mechanism for this important recruitment step is poorly understood, as is the function of adaptor proteins during fission complex formation and membrane severing. In this thesis, we studied DRP membrane recruitment and adaptor functions using the yeast mitochondrial fission machinery including the DRP Dnm1, the adaptor Mdv1, and the membrane anchor Fis1. To initiate mitochondrial fission, Dnm1 is recruited to the mitochondria via a direct interaction with Mdv1, which binds Fis1 on the mitochondrial outer membrane. As described in Chapter 2, we identified a novel motif in the Dnml Insert B domain that is essential for the interaction of Dnml with Mdvl. Mutations in this conserved motif severely impaired Dnml-Mdvl interaction and mitochondrial fission. Suppressor mutations in the Mdvl P-propeller domain rescued defects caused by the Insert B mutations. The positions of these suppressor mutations define potential binding sites for Insert B on the Mdvl P-propeller. In Chapter 3, we demonstrated that Mdvl plays dual functions, as an adaptor and in scaffolding, during fission complex formation. Mdvl dimerizes via a 92A coiled-coil, which determines the optimal architecture of fission complexes. Together, these studies provide mechanistic insights regarding DRP membrane recruitment and the functions of adaptors in DRP-mediated membrane fission. Although the sequence of Insert B and the adaptors are varied among DRPs, we believe that they are functionally conserved and may have co-evolved to meet the requirements of specific fission events in different tissues and cell types.