Synapsin I  Synapsin I, is the collective name for Synapsin Ia and Synapsin Ib, two nearly identical phosphoproteins that in humans are encoded by the "SYN1" gene. In its phosphorylated form, Synapsin I may also be referred to as phosphosynaspin I. Synapsin I is the first of the proteins in the synapsin family of phosphoproteins in the synaptic vesicles present in the central and peripheral nervous systems. Synapsin Ia and Ib are close in length and almost the same in make up, however, Synapsin Ib stops short of the last segment of the C-terminal in the amino acid sequence found in Synapsin Ia. The synapsin I protein is a member of the synapsin family that are neuronal phosphoproteins which associate with the cytoplasmic surface of synaptic vesicles. Family members are characterized by common protein domains, and they are implicated in synaptogenesis and the modulation of neurotransmitter release, suggesting a potential role in several neuropsychiatric diseases. The phosphoprotein plays a role in regulation of axonogenesis and synaptogenesis. The protein serves as a substrate for several different protein kinases and phosphorylation may function in the regulation of this protein in the nerve terminal. Synapsin I is found in two isoforms of the protein, Synapsin Ia and Synapsin Ib, with Synapsin Ib being a slightly shorter version of the protein. Both Synapsin I proteins are highly basic with a pI in the range of 10.3 and 10.2, respectively. Both isoforms are phosphorylated at identical locations within their protein sequences at the same three serine residues. Synapsin I phosphoproteins make up approximately 6% of the total protein in synaptic vesicles. Among bovine, rat, and human it has been shown to be 95% homologous, with the central 'C' domain evolutionarily conserved. This phosphoprotein is loosely associated with the vesicular membrance and is easily dissociated by treatment with a salt, versus a detergent being required for its removal from the membrane. Synapsin I proteins are made up of a globular portion at the N-terminal and an elongated C-terminal domain, rendering them largely elongated. Synapsin Ib has the same protein domains as synapsin Ia, however synapsin Ib lacks the last C-terminal segment, making it slightly shorter in its elongated domain. 706 amino acids comprise synapsin Ia, and starting from the N-terminal, the same first 670 amino acids comprise synapsin Ib.