Model of p27 function in terminal erythroid differentiation.

p27 protein accumulates in cyclin D/cdk4 complexes during differentiation without inhibiting kinase activity. When cyclin D/cdk4 complexes become saturated with p27, p27 accumulates in cyclin E/cdk2 complexes and inhibits cdk2 kinase. Complete loss of cdk2 kinase activity requires loss of cyclin E from the complex. Loss of cyclin D/cdk4 kinase activity occurs by an unknown mechanism.