Mammalian Abp1, a signal-responsive F-actin-binding protein, links the actin cytoskeleton to endocytosis via the GTPase dynamin

The actin cytoskeleton has been implicated in endocytosis, yet few molecula r links to the endocytic machinery have been established. Here we show that the mammalian F-actin-binding protein Abp1 (SH3P7/HIP-55) can functionally link the actin cytoskeleton to dynamin, a GTPase that functions in endocyt osis. Abp1 binds directly to dynamin in vitro through its SH3 domain, Coimm unoprecipitation and colocalization studies demonstrated the in vivo releva nce of this interaction. In neurons, mammalian Abp1 and dynamin colocalized at actin-rich sites proximal to the cell body during synaptogenesis In fib roblasts mAbp1 appeared at dynamin-rich sites of endocytosis upon growth fa ctor stimulation. To test whether Abp1 functions in endocytosis, we overexp ressed several Abp1 constructs in Cos-7 cells and assayed receptor-mediated endocytosis. While overexpression of Abp1's actin-binding modules did not interfere with endocytosis, overexpression of the SH3 domain led to a poten t block of transferrin uptake. This implicates the Abp1/dynamin interaction in endocytic function. The endocytosis block was rescued by cooverexpressi on of dynamin. Since the addition of the actin-binding modules of Abp1 to t he SH3 domain construct also fully restored endocytosis, Abp1 may support e ndocytosis by combining its SH3 domain interactions with cytoskeletal funct ions in response to signaling cascades converging on this linker protein.