Retinoblastoma protein phosphorylation does not require activation of p34CDC2 protein kinase

Of the many intracellular events that occur after mitogenic stimulation of cells, the phosphorylation of the retinoblastoma protein (RB) in early G1-phase appears to play a pivotal role in controlling cell-cycle progression. RB phosphorylation results in release from a proliferative block imposed by hypophosphorylated RB. Several investigators have presented evidence, using models produced in vitro, that the serine kinase p34CDC2 phosphorylates RB and is responsible for regulating RB phosphorylation. Using human T-cells as a model, we show that lectin treatment of resting T-cells results in detectable RB phosphorylation by 24 h after treatment. Further, using immunoprecipitation and immunoblotting, no detectable p34CDC2 could be seen until 48 h after lectin stimulation. Analysis of the relative histone H1 activity of p34CDC2, purified by immunoprecipitation, revealed that RB phosphorylation does not parallel increases in p34CDC2 activity as T-cells progress into S-phase, supporting the contention that p34CDC2 activation as a histone H1 kinase is not a critical regulator of RB phosphorylation. Further treatment of activated T-cells, arrested in G1-phase, with interleukin 2 results in a 95% increase in RB phosphorylation within 4 h with no detectable increase in the histone H1 kinase activity of p34CDC2. Together, these data suggest that p34CDC2 activation is not required for early cell-cycle phosphorylation of RB.