Cloning from insulinoma cells of synapsin I associated with insulin secretory granules

Synapsin I is a synaptic vesicle-associated protein involved in neurotransm itter release. The functions of this protein are apparently regulated by Ca 2+/calmodulin-dependent protein kinase II (CaM kinase II). We reported evid ence for CaM kinase II and a synapsin I-like protein present in mouse insul inoma MIN6 cells (Matsumoto, K., Fukunaga, K., Miyazaki, J., Shichiri, M., and Miyamoto, E. (1995) Endocrinology 136, 3784-3793), Phosphorylation of t he synapsin I-like protein in these cells correlated with the activation of CaM kinase II and insulin secretion. In the present study, we screened the MING cDNA library with the full-length cDNA probe of rat brain synapsin Ia and obtained seven positive clones; the largest one was then sequenced. Th e largest open reading frame deduced from the cDNA sequence of 3695 base pa irs encoded a polypeptide of 670 amino acids, which exhibited significant s equence similarity to rat synapsin Ib, The cDNA contained the same sequence as the first exon of the mouse synapsin I gene. These results indicate tha t synapsin Ib is present in MING cells. Synapsin I was expressed in normal rat islets, as determined by reverse transcriptase-polymerase chain reactio n analysis. Immunoblot analysis after subcellular fractionation of MING cel ls demonstrated that synapsin Ib and delta subunit of CaM kinase II co-loca lized with insulin secretory granules. By analogy concerning regulation of neurotransmitter release, our results suggest that phosphorylation of synap sin I by CaM kinase II may induce the release of insulin from islet cells.