A 40-KDA MYELIN BASIC-PROTEIN KINASE, DISTINCT FROM ERK1 AND ERK2, ISACTIVATED IN MITOTIC HELA-CELLS

Mitotic HeLa cells showed an increased phosphorylation activity toward s myelin basic protein compared to cells in G1 or S phases. Further in vestigation using renaturation gels revealed that, in mitotic cell lys ates, a protein with an apparent molecular mass of around 40 kDa phosp horylates myelin basic protein. This kinase is active early in mitosis , but is then downregulated concomitantly with p34(cdc2) kinase as mit osis proceeds, its activity decreasing to basal levels by early GI. Th e molecular mass of the kinase suggested that it might be one of the h uman homologues of rat erk1 or erk2. However, antibodies raised agains t C-terminal sequences of erk1 and erk2 failed to immunoprecipitate re naturable kinase activity from mitotic lysates. In addition, in immuno blots erk1 and erk2 failed to show the well established changes in ele ctrophoretic migration that are consequences of their activation. Thes e data indicate that these two mitogen-activated protein (MAP) kinases are not stimulated during HeLa cell mitosis and indicate that the 40- kDa kinase is either a new member of the MAP kinase family or it is a novel mitotic kinase that has not yet been described.