REGULATION OF MUNC-18 SYNTAXIN 1A INTERACTION BY CYCLIN-DEPENDENT KINASE-5 IN NERVE-ENDINGS

The Munc-18-syntaxin 1A complex has been postulated to act as a negati ve control on the regulated exocytotic process because its formation b locks the interaction of syntaxin with vesicle SNARE proteins, However , the formation of this complex is simultaneously essential for the fi nal stages of secretion as evidenced by the necessity of Munc-18's hom ologues in Saccharomyces cerevisiae (Sec1p), Drosophila (ROP), and Cae norhabditis elegans (Unc-18) for proper secretion in these organisms, As such, any event that regulates the interaction of these two protein s is important for the control of secretion, One candidate for such re gulation is cyclin-dependent kinase 5 (Cdk5), a member of the Cdc2 fam ily of cell division cycle kinases that has recently been copurified w ith Munc-18 from rat brain, The present study shows that Cdk5 bound to its neural specific activator p35 not only binds to Munc-18 but utili zes it as a substrate for phosphorylation, Furthermore, it is demonstr ated that Munc-18 that has been phosphorylated by Cdk5 has a significa ntly reduced affinity for syntaxin Ik Finally, it is shown that Cdk5 c an also bind to syntaxin 1A and that a complex of Cdk5, p35, Munc-18, and syntaxin 1A can be fashioned in the absence of ATP and promptly di sassembled upon the addition of ATP, These results suggest a model in which p35-activated Cdk5 becomes localized to the Munc-18-syntaxin 1A complex by its affinity for both proteins so that it may phosphorylate Munc-18 and thus permit the positive interaction of syntaxin 1A with upstream protein effecters of the secretory mechanism.