How do synaptic vesicles move towards the presynaptic plasma membrane, fuse with that membrane, and release their contents during synaptic transmission? The answers to these questions at the molecular level are just beginning to be understood. Synapsin I is a neuron specific phosphoprotein that is associated with the cytoplasmic surface of synaptic vesicles. During synaptic transmission, the translocation of the synaptic vesicles to the presynaptic membrane of the neuron is thought to be mediated through changes in the phosphorylation state of synapsin I. It has been suggested that synapsin I is a spectrin binding protein related to the erythrocyte cytoskeletal protein 4.1, which binds to the terminal ends of the erythrocyte spectrin tetramer. The interaction of synapsin I (through brain spectrin) with the neuronal cytoskeleton may be essential for regulating the movement of synaptic vesicles towards the presynaptic plasma membrane. In addition, we have identified another protein in brain that is immunologically and structurally more closely related to erythrocyte 4.1 than is synapsin I. This protein, termed amelin, is localized in the cell body and dendrites of the neuron, whereas synapsin I is found exclusively in the synaptic terminals, suggesting that there is a family of erythrocyte 4.1 related proteins present in brain with distinct subcellular distribution and functions.
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