The regulated expression of cell cycle-related proteins as B-lymphocytes enter and progress through the G₁ cell cycle stage following delivery of complete versus partial activation stimuli

Resting B-cells lack both cyclins D an E while constitutively expressing low levels of cdk4 and cdk2. B-cells receiving a complete growth stimulus express cyclin D2 by 10 hr and cyclin E by 10-24 hr poststimulation while increasing their protein levels of cdk4 and cdk2. B-cells receiving partial growth stimuli move into G₁ without passing the G₁ restriction point and transiently increase cyclin D2 mRNA levels without accumulating cyclin D2 protein. In the absence of cyclin D2 accumulation, cdk4 is not activated, and cyclin E is not expressed. These results suggest that signals responsible for moving B-cells through the G₁ restriction point impact at the level of cyclin D2 protein accumulation. The possible implications of these results are discussed.