Microtubules and Src homology 3 domains stimulate the dynamin GTPase via its C-terminal domain

Dynamin is a 100-kDa GTPase that plays a critical role in the initial stages of endocytosis. Dynamin binds to microtubules, which potently stimulate its GTPase activity. Binding to Src homology 3 (SH3) domains of proteins involved in signal transduction has also recently been reported. In the present study, the protein was digested with a variety of proteases to define its functional domains. Limited digestion with papain split the protein into an approximately 7- to 9-kDa microtubule-binding fragment and a 90-kDa nonbinding fragment. Immunoblotting with an antibody to the C-terminal 20 amino acids of rat dynamin showed the small fragment to derive from the C-terminal end of the polypeptide. Microtubule-activated GTPase activity, but not basal GTPase activity, was abolished by papain digestion, identifying the basic, proline-rich C-terminal region of dynamin as an important regulatory site. Bacterially expressed growth factor receptor-bound protein 2 (GRB2) and the SH3 domain of c-Src were also found to stimulate GTPase activity, although to a lesser extent than microtubules. Stimulation of GTPase activity by the recombinant proteins was similarly abolished by papain digestion. These results identify the basic, proline-rich C-terminal region of dynamin as the binding site for both microtubules and SH3 domains and demonstrate an allosteric interaction between this region of the molecule and the N-terminal GTPase domain.