<p>The synapsins are a family of neuron-specific phosphoproteins that coatsynaptic vesicles and are involved in the binding between these vesiclesand the cytoskeleton (including actin filaments). The family comprises 5homologous proteins Ia, Ib, IIa, IIb and III. Synapsins I, II, and III areencoded by 3 different genes. The a and b isoforms of synapsin I and II aresplice variants of the primary transcripts [<cite idref="PUB00018754"/>].</p><p>Synapsin I is mainly associated with regulation of neurotransmitter releasefrom presynaptic neuron terminals [<cite idref="PUB00053858"/>]. Synapsin II, as well as being involved in neurotransmitter release, has a role in the synaptogenesis and synaptic plasticity responsible for long term potentiation [<cite idref="PUB00018755"/>]. Recent studies implicate synapsin III with a developmental role in neurite elongation and synapse formation that is distinct from the functions of synapsins I and II [<cite idref="PUB00053859"/>].</p><p>Structurally, synapsins are multidomain proteins, of which 3 domains arecommon to all the mammalian forms. The N-terminal `A' domain is ~30 residueslong and contains a serine residue that serves as an acceptor site for protein kinase-mediated phosphorylation. This is followed by the `B' linkerdomain, which is ~80 residues long and is relatively poorly conserved.Domain `C' is the longest, spanning approximately 300 residues. This domainis highly conserved across all the synapsins (including those fromDrosophila) and is possessed by all splice variants. The remaining sixdomains, D-I, are not shared by all the synapsins and differ both betweenthe primary transcripts and the splice variants.</p><p>This entry represents a highly conserved stretch of 11 residues located in the centre of the 'C' domain.</p>