In Drosophila, Amphiphysin exhibits strong postsynaptic expression, whereas in mammalian tissue, it is highly enriched in the brain. However, one or more splice forms of mammalian amphiphysin 2 that lack a consensus clathrin-binding motif are also highly expressed in skeletal muscle and are similar to the amphiphysin splice form (Amph 2-6) cloned from kidney. Using pull-down assays, the binding was tested of glutathione-S-transferase (GST) fusions of the N- and C-terminal domains of Drosophila Amphiphysin and rat amphiphysin 2-1 (Amph 2-1), as well as that of whole rat Amph 2-6, to rat brain, rat muscle, and whole Drosophila protein extracts. The N terminus of the brain-enriched form, Amph 2-1, has a clathrin-binding consensus sequence and binds clathrin. Amph 2-6 and Drosophila Amphiphysin lack this sequence, failed to bind clathrin, and are therefore unlikely to participate in clathrin-mediated endocytosis. The SH3 domains of rat amphiphysin 2 and Drosophila Amphiphysin bind to rat brain dynamin, but not to Drosophila dynamin. The latter is probably a consequence of the lack of conservation of the consensus amphiphysin SH3-binding motif (PxRPxR) in Drosophila dynamin. In contrast, the SH3 domains of Dynamin associated protein 160 (DAP160), the Drosophila intersectin homolog, bind strongly to Drosophila dynamin. The failure of Drosophila Amphiphysin to interact significantly with Drosophila dynamin also argues against an endocytic role in Drosophila (Razzaq, 2001).
Since the N terminus of rat amphiphysin 1 has the ability to tubulate lipids in vitro (Takei, 1999, tests were made to see whether rat Amph 2-6 and Drosophila Amphiphysin also had this ability. Both full-length Amph 2-6 and the Drosophila Amphiphysin N-terminal domain readily promote the formation of tubular projections from the surface of liposomes within the time necessary for their preparation for electron microscopy. The Amphiphysin protein forms the scaffold for these lipid projections. The diameters of the T-tubules in IFM electron micrographs are, on average, wider than the projections generated in vitro from Folch lipids. This difference may be a consequence of the diameter in vivo being governed by different lipid contents, as well as by additional proteins present in T-tubule projections, such as Dlg. In conclusion, both rat Amph 2-6 and Drosophila Amphiphysin have the potential to initiate tubule extensions of lipid membranes similar to those seen within the reticular T-tubule network of muscles (Razzaq, 2001).
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