Calpain-dependent proteolytic cleavage of the p35 cyclin-dependent kinase 5 activator to p25

Cyclin-dependent kinase 5 (CDK5) is a unique CDK, the activity of which can be detected in postmitotic neurons. To date, CDK5 purified from mammalian brains has always been associated with a truncated form of the 35-kDa major brain specific activator (p35, also known as nck5a) of CDK5, known as p25. In this study,we report that p35 can be cleaved to p25 both in nitro and i n vivo by calpain. In a rat brain extract, p35 was cleaved to p25 by incuba tion with Ca2+. This cleavage was inhibited by a calpain inhibitor peptide derived from calpastatin and was ablated by separating the p35.CDK5 from ca lpain by centrifugation. The p35 recovered in the pellet after centrifugati on could then be cleaved to p25 by purified calpain. Cleavage of p35 was al so induced in primary cultured neurons by treatment with a Ca2+ ionophore a nd Ca2+ and inhibited by calpain inhibitor I. The cleavage changed the solu bility of the CDK5 active complex from the particulate fraction to the solu ble fraction but did not affect the histone H1 kinase activity. Increased c leavage was detected in cultured neurons undergoing cell death, suggesting a role of the cleavage in neuronal cell death.