DP1 phosphorylation in multimeric complexes: weaker interaction with cyclin A through the E2F1 cyclin A binding domain leads to more efficient phosphorylation than stronger interaction through the p107 cyclin A binding domain.

Stable enzyme-substrate interaction has been recognized as a major mechanism underlying the substrate preferences of cyclin-dependent kinases (Cdks). To learn the relationship between stability of physical association and efficiency of phosphorylation, we studied DP1 phosphorylation by cyclin A-Cdk2 in multiprotein complexes. When DP1 was connected to cyclin A-Cdk2 through E2F4 and p107, its phosphorylation was very inefficient, although its association with cyclin A-Cdk2 was stable. In contrast, DP1 was efficiently phosphorylated when weakly connected to cyclin A-Cdk2 via E2F1 or E2F4 with a fused cyclin A binding domain of E2F1. The transactivation activity of E2F4-DP1 heterodimers was reduced when DP1 was phosphorylated, while a phosphorylation deficient mutant of DP1 resisted this down-regulation. Phosphorylation and functional regulation of DP1 were not due to nuclear localization. Thus, stronger physical association between the kinase and the substrate does not necessarily lead to more efficient phosphorylation than weaker interaction does.