CELL CYCLE-SPECIFIC INDUCTION OF CDK2 EXPRESSION IN B-LYMPHOCYTES FOLLOWING ANTIGEN RECEPTOR CROSS-LINKING

The ligation of membrane Ig (mig) on quiescent primary B lymphocytes by mitogenic concentrations of anti-IgM antibodies leads to cell cycle progression. The level of cyclin-dependent kinase 2 (Cdk2) expression was found to be restricted to specific phases of the cell cycle in primary cultures of murine B lymphocytes. Resting G(0) phase, G(1) phase, or B cells arrested near the G(1)/S boundary by hydroxyurea contained no detectable Cdk2 protein or associated histone H1 kinase activity. In contrast, B cell entry into S phase was accompanied by an induction in the expression of cellular Cdk2 as judged by immunoblotting of B cell lysates with anti-Cdk2 antibodies. Concomitant with S phase entry was the detection of anti-Cdk2-specific immunoprecipitable histone H1 kinase activity. Further analysis revealed that the amount of cyclin A protein also oscillated during cell cycle, appearing initially in G(1) phase B cells. Cyclin A was found to be associated with Cdk2 in B cells during S phase progression. These results indicate that cross-linking of mig on primary B lymphocytes results in the ''downstream'' catalytic activation of Cdk2. The timing of Cdk2 expression and its association with cyclin A suggests that Cdk2 may not be involved in the decision to enter S phase, but rather may provide a role in the maintenance of S phase progression or in preparing B cells to enter M phase.