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Synapsins

 

The synapsins are a family of neuronal ABPs that also bind vesicles.  It is generally beleived that their function is to bind the vesicles to the actin cytoskeleton to regulate vesicle docking and transmitter release.  Two major types, synapsin I and II exist and each of these are present as two isoforms so that the full complement is Synapsin Ia, Ib, IIa and IIb. They are composed of a number of recognisable domains.  All synapsins contain an N terminal "A" domain, followed by a "B" domain followed in turn by a "C" domain, after which considerable diversity is found.  Synapsin I has been shown to bind and bundle actin in a phosphorylation dependent manner (Bähler & Greengard, 1987).  Synapsin I has also been found to have nucleating activity, that is to enhance spontaneous actin polymerization by the formation of "seeds".   More recently synapsin IIa has a also been shown to bundle actin microfilaments , even more potently than Ia  (Chilcote et al, 1994).   No homology between the synapsins and other actin binding proteins is believed to exist (former reports were based on erroneous sequence data).

References:-

Bähler, M. &  Greengard, P. (1987). “Synapsin I bundles F-actin in a phosphorylation-dependent manner.” Nature 326: 704-707.

Ceccaldi, P.-E., Grohovaz, F., Benfenati, F., Chieregatti, E., Greengard, P., & Valtorta, F. (1995). “Dephosphoryated synapsin I anchors synaptic vesicles to actin cytoskeleton: an analysis by videomicroscopy.” J.Cell Biol. 128(5): 905-912.

Chilcote, T. J., Siow, Y.L., Schaeffer, E., Greengard, P., & Thiel, G. (1994). “Synapsin IIa bundles actin filaments.” J. Neurochem. 63: 1568-1571.

Onofri, F.,  Giovedi, S., Kao, H.-T., Valtorta, F., Borbone, L.B., De Camilli, P., Greengard, P. & Benfenati, F. (2000). “Specificity of the binding of synapsin I to Src homology 3 domains.” J.Biol.Chem. 275(38): 29857-29867.

 
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