Identification of an alternatively spliced avian member of the synaptophysin gene family.

Synaptic vesicle membrane proteins are important in the release of neurotransmitters and as markers of presynaptic differentiation in neurons, and the synaptophysins are a major class of synaptic vesicle proteins. By low stringency screening of a chick brain cDNA library with a rat synaptophysin probe, we have isolated cDNAs that encode a novel member of the synaptophysin/synaptoporin family. Two different protein-coding forms of the cDNA were found, apparently generated through alternative splicing of a single gene. The deduced proteins, called synaptophysin IIa and synaptophysin IIb, share 258 amino acids (starting from position 10 in IIa and position 30 in IIb), that are most closely related to the rat synaptoporin sequence. The N-terminal sequence of IIa is similar to that of rat synaptoporin, and the N-terminal sequence of IIb is similar to that of rat synaptophysin. Northern blot analysis and nuclease protection experiments demonstrate that IIa and IIb are expressed in a variety of brain regions, the spinal cord, and dorsal root ganglia, but not in non-neuronal tissues. Further, the two splice variants are differentially distributed. In most brain regions the IIb form predominates, and the cerebellum appears to express only the IIb form, but the IIa form is relatively elevated in peripheral neurons. Western blot analysis with an antibody to a synthetic peptide common to both forms demonstrates the expression of synaptophysin II as a 39 kDa protein, apparently distinct from synaptophysin (40 kDa). The results suggest that the regulation and function of the synaptophysin gene family is more complex than had been appreciated.