Functional linkage of Retinoblastoma protein family turnover and potency during mouse embryonic stem cell development

Abstract

In embryonic stem (ES) cells, the Retinoblastoma family of proteins is held in a biochemically inactive state through robust cyclin/cdk‐mediated phosphorylation. However, at the onset of differentiation, diminished cyclin/cdk activity contributes to the establishment of functional RB/p107/p130 pathways and regulated cell cycle control. We report that the invocation of pocket protein function during ES cell differentiation is also correlated with decreased repressor abundance, suggesting that repressor potency is linked to turnover. Indeed, wild type p107 levels were diminished by pharmacological inhibition of cyclin/cdk activity while mutant p107 lacking an evolutionarily conserved instability element within its C‐terminal regulatory domain was refractory to cyclin/cdk modulation. Select amino acid substitutions within this instability element increased p107 abundance and phosphorylation, but crippled p107 repressor function. The instability elements of both p107 and p130 can direct proteasome‐mediated turnover when fused to heterologous reporter proteins indicating that these elements can function as autonomous degrons. Importantly, mutations within the p130 degron have been observed in human lung carcinoma patients indicating an important role for the regulation of degron function in human disease. This research was supported by the National Institutes of Health.