Structural basis of dynamin-mediated membrane fission actin bundling and interaction with binding partners.

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Project Summary/Abstract Dynamin GTPases have critical roles in mediating endocytosis by wrapping around the neck of budding vesicles to catalyze membrane fission necessary for the release of nascent vesicles from the plasma membrane. Recently, we discovered a novel role for dynamin in bundling numerous actin filaments, which has implications for actin-mediated processes, such as cell-cell fusion and migration. While structural and biophysical studies have elucidated the mechanism of dynamin assembly and constriction of membranes, several unanswered questions remain including how dynamin is actually organized and mediates fission within cells, how dynamin forms the final pre-fission state where it wraps around lipid tubules in a superconstricted state, how dynamin binds substrates via the proline rich domain (PRD), and how substrate binding regulates dynamin activity. The long-term objective of this application is to elucidate the mechanism of dynamin-mediated membrane fission from in vitro and in vivo studies, and to define the mechanism of dynamin interaction with actin and SH3 domain- containing proteins that recruit dynamin to sites of endocytosis. These objectives will be addressed by the following specific aims: (1) determine the atomic model of dynamin in the superconstricted prefission state and define the structure of the PRD; (2) investigate the assembly of the dynamin helical polymer on membranes within cells; and (3) Elucidate the mechanism of PRD interaction with actin filaments and SH3 domain-containing binding partners. The rationale for these aims is that: (1) there is no atomic model describing the structural basis by which dynamin constricts membranes to 3.4 nm in the superconstricted state where spontaneous hemifission and membrane fission occurs; (2) how dynamin is actually organized in cells has not been reported; and (3) the structure of the critical PRD and how it binds dynamin substrates including actin, amphiphysin and intersectin is unknown. The research design and methods are as follows : Aim 1, Apply cryo-electron microscopy (cryo-EM) to determine the structure of full-length dynamin (containing the PRD) organized around lipid tubules in the superconstricted state; Aim 2, obtain cryo-electron tomograms and subtomogram averages of dynamin within cells transfected with a GTPase-deficient dynamin mutant which delays membrane fission and extends the lifetime of dynamin helices on cellular membranes; Aim 3, obtain the cryo-EM structure of actin filaments decorated with PRD from dynamin, as well as complexes of dynamin/amphiphysin and dynamin/intersectin. This work is of