AMPHIPHYSIN HETERODIMERS - POTENTIAL ROLE IN CLATHRIN-MEDIATED ENDOCYTOSIS

Amphiphysin (Amph) is a src homology 3 domain-containing protein that has been implicated in synaptic vesicle endocytosis as a result of its interaction with dynamin. In a screen for novel members of the amphip hysin family, we identified Amph2, an isoform 49% identical to the pre viously characterized Amph1 protein. The subcellular distribution of t his isoform parallels Amph1, both being enriched in nerve terminals. L ike Amph1, a role in endocytosis at the nerve terminal is supported by the rapid dephosphorylation of Amph2 on depolarization. Importantly, the two isoforms can be coimmunoprecipitated from the brain as an equi molar complex, suggesting that the two isoforms act in concert. As det ermined by cross-linking of brain extracts, the Amph1-Amph2 complex is a 220- to 250-kDa heterodimer. COS cells transfected with either Amph 1 or Amph2 show greatly reduced transferrin uptake, but coexpression o f the two proteins rescues this defect, supporting a role for the hete rodimer in clathrin-mediated endocytosis. Although the src homology 3 domains of both isoforms interact with dynamin, the heterodimer can as sociate with multiple dynamin molecules in vitro and activates dynamin 's GTPase activity. We propose that it is an amphiphysin heterodimer t hat drives the recruitment of dynamin to clathrin-coated pits in endoc ytosing nerve terminals.