SV2proteoglycan: a potential synaptic vesicle transporter and nerve terminal extracellular matrix receptor

SV2Proteoglycan (SV2pg) is a specific component of small clear synaptic vesicles. It is a keratan sulfate proteoglycan with oligosaccharide side chains N-linked to a core protein of 80 kDa. Two glycosylated forms, H and L, are present in synaptic vesicles. The amino acid sequence suggests that SV2pg contains 12 transmembrane domains and is homologous to bacterial and eukaryotic sugar transporters. Although its structure suggests that SV2pg is a vesicular transporter, what it transports is unknown. SV2pg is probably not a neurotransmitter transporter, since that function resides in an unrelated family of synaptic vesicle proteins. In addition to its vesicular function, SV2pg may have a secondary function as an extracellular matrix (ECM) receptor on the nerve terminal surface. At the electric organ synapse, which is closely related to the neuromuscular junction. SV2pg is bound to laminin, a component of the synaptic ECM. SV2pg is only associated with laminin on the nerve terminal surface, not in the synaptic vesicle. Studies on synaptogenesis during motor nerve regeneration have suggested that nerve terminal ECM receptors play an important role in synaptic recognition.