Synapsin I is a microtubule-bundling protein.
Synapsin I, a synaptic vesicle protein, is thought to be involved in the regulation of neurotransmission through its phosphorylation by the cyclic AMP-dependent and Ca2+/calmodulin-dependent protein kinases which become activated upon depolarization of nerve endings. However, despite its recent characterization as a spectrin-binding protein immunologically related to erythrocyte protein 4.1, other interactions of synapsin I with structural proteins remain unknown. We report here that synapsin I can co-cycle with microtubules through three cycles of warm polymerization and cold depolymerization. Synapsin I binds saturably to microtubules stabilized by taxol, with an estimated dissociation constant (Kd) of 4.5 microM and a stoichiometry of 1.2 mol of synapsin binding sites per mol tubulin dimer. Synapsin I also increases the turbidity of tubulin solutions at 37 degrees C, but without causing detectable alterations in the critical concentration required for polymerization. Mixtures of synapsin I and tubulin observed by negative stain electron microscopy contain bundles of microtubules, accounting for the effect of synapsin I on tubulin turbidity. Synapsin I is thus a candidate to mediate or regulate the interaction of synaptic vesicles with microtubules.
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Baines
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Bennett
D000470:Alkaloids / Q000494:pharmacology; D000818:Animals; D001370:Axonal Transport; D001923:Brain Chemistry; D002417:Cattle; D008854:Microscopy, Electron; D008869:Microtubule-Associated Proteins / Q000378:metabolism*; D009419:Nerve Tissue Proteins / Q000378:metabolism*; D017239:Paclitaxel; D011485:Protein Binding; D016704:Synapsins; D013572:Synaptic Vesicles; D014404:Tubulin / Q000378:metabolism
D000470:Alkaloids; D008869:Microtubule-Associated Proteins; D009419:Nerve Tissue Proteins; D016704:Synapsins; D014404:Tubulin; D017239:Paclitaxel