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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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