InterPro

The synapsins are a family of neuron-specific phosphoproteins that coat synaptic vesicles and are involved in the binding between these vesicles and the cytoskeleton (including actin filaments). The family comprises 5 homologous proteins Ia, Ib, IIa, IIb and III. Synapsins I, II, and III are encoded by 3 different genes. The a and b isoforms of synapsin I and II are splice variants of the primary transcripts [PMID: 10940454].

Synapsin I is mainly associated with regulation of neurotransmitter release from presynaptic neuron terminals [PMID: 2859595]. Synapsin II, as well as being involved in neurotransmitter release, has a role in the synaptogenesis and synaptic plasticity responsible for long term potentiation [PMID: 7777057]. 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 [PMID: 10804215].

Structurally, synapsins are multidomain proteins, of which 3 domains are common to all the mammalian forms. The N-terminal `A' domain is ~30 residues long and contains a serine residue that serves as an acceptor site for protein kinase-mediated phosphorylation. This is followed by the `B' linker domain, which is ~80 residues long and is relatively poorly conserved. Domain `C' is the longest, spanning approximately 300 residues. This domain is highly conserved across all the synapsins (including those from Drosophila) and is possessed by all splice variants. The remaining six domains, D-I, are not shared by all the synapsins and differ both between the primary transcripts and the splice variants.

This entry represent the ATP-grasp fold found in synapsins, which is responsible for Ca dependent ATP binding.