A CDC2-RELATED KINASE PSSALRE CDK5 IS HOMOLOGOUS WITH THE 30 KDA SUBUNIT OF TAU-PROTEIN KINASE-II, A PROLINE-DIRECTED PROTEIN-KINASE ASSOCIATED WITH MICROTUBULE/

We previously reported that tau protein kinase II (TPKII) from bovine brain was composed of 30 kDa and 23 kDa subunits. The 30 kDa subunit o f TPKII can be regarded as a catalytic subunit because of its ATP-bind ing activity. Antibodies directed against TPKII-phosphorylated tau als o reacted with tau phosphorylated by cdc2 kinase obtained from starfis h oocytes, indicating that TPKII and cdc2 kinase phosphorylate the sam e sites. We determined the amino acid sequence of the 30 kDa subunit a nd found it to be homologous with a cdc2-related kinase, PSSALRE/cdk5. Moreover, an antibody against PSSALRE/cdk5 reacted with the 30 kDa su bunit. These results indicate that the 30 kDa subunit of TPKII is bovi ne homologue of PSSALRE/cdk5. Expression of the 30 kDa subunit mRNA wa s enhanced in juvenile rat brain. This result supports our previous hy pothesis that the kinase works actively in juvenile brain.