Suppression of calpain-dependent cleavage of the CDK5 activator p35 to p25 by site-specific phosphorylation

Cdk5 is a proline-directed Ser/Thr protein kinase predominantly expressed in postmitotic neurons together with its activator, p35. N-terminal truncation of p35 to p25 by calpain results in deregulation of Cdk5 and contributes to neuronal cell death associated with several neurodegenerative diseases. Previously we reported that p35 occurred as a phosphoprotein, phospho-p35 levels changed with neuronal maturation, and that phosphorylation of p35 affected its vulnerability to calpain cleavage. Here, we identify the p35 residues Ser ⁸ and Thr¹³⁸ as the major sites of phosphorylation by Cdk5. Mutagenesis of these sites to unphosphorylatable Ala increased susceptibility to calpain in cultured cells and neurons while changing them to phosphomimetic glutamate-attenuated cleavage. Furthermore, phosphorylation state-specific antibodies to these sites revealed that Thr¹³⁸ was dephosphorylated in adult rat, although both Ser⁸ and Thr ¹³⁸ were phosphorylated in prenatal brains. In cultured neurons, inhibition of protein phosphatases converted phosho-Ser⁸ p35 to dual phospho-Ser⁸/Thr¹³⁸ p35 and conferred resistance to calpain cleavage. These results suggest phosphorylation of Thr¹³⁸ predominantly defines the susceptibility of p35 to calpain-dependent cleavage and that dephosphorylation of this site is a critical determinant of Cdk5-p25-induced cell death associated with neurodegeneration.