AN ISOFORM OF THE NEURONAL CYCLIN-DEPENDENT KINASE-5 (CDK5) ACTIVATOR

Neuronal Cdc2-like kinase is a heterodimer of Cdk5 and a 25-kDa subuni t that is derived from a 35-kDa brain- and neuron-specific protein cal led the neuronal Cdk5 activator (p35/p25(nck5a)) (Lew, J., Huang, Q.-Q ., Qi, Z., Winkfein, R. J., Aebersold, R., Hunt, T., and Wang, J. H. ( 1994) Nature 371, 423-426; Tsai, L. H., Delalle, I., Caviness, V. S., Jr., Chae, T., and Harlow, E. (1994) Nature 371, 419-423). Upon screen ing of a human hippocampus library with a bovine Nck5a cDNA, we uncove red a distinct clone encoding a 39-kDa isoform of Nck5a. The isoform, designated the neuronal Cdk5 activator isoform (p39(nck5ai)), showed a high degree of sequence similarity to p35(nck5a) with 57% amino acid identity. Northern blot analysis detected its mRNA transcript in bovin e and rat cerebrum and cerebellum, but not in any other rat tissues ex amined. In situ hybridization showed that Nck5ai was enriched in CA1 t o CA3 of the hippocampus, but absent in the fimbria of hippocampal for mation. Among seven cell lines in proliferating cultures, only PC12 an d N2A, two cell lines capable of differentiating into neuron-like cell s, were found to contain Nck5ai mRNA. A 30-kDa truncated form of Nck5a i expressed as a glutathione S-transferase fusion protein in Escherich ia coli] was found to associate with Cdk5 to form an active Cdk5 kinas e. Thus, the isoform shares many common characteristics with p35(nck5a ), including Ckd5 activating activity and brain- and neuron-specific e xpression. Both proteins show limited sequence homology to cyclins, su ggesting that they define a new family of cyclin-dependent kinase-acti vating proteins.