A Unique Domain of pRb2/p130 Acts as an Inhibitor of Cdk2 Kinase Activity*

The Cdk2 kinase has long been known to be involved in the progression of mammalian cells past the G1phase restriction point and through DNA replication in the cell cycle. The Rb family of proteins, consisting of pRb, p107, and pRb2/p130, has also been shown to monitor progression of G1phase, mostly through their interaction with E2F family members. p107 is able to inhibit Cdk2 kinase activity through this interaction via a p21-related domain present in the C terminus of the protein. We show here that pRb2/p130 also possesses this activity, but through a separate domain. Moreover, we correlate the increased expression of pRb2/p130 during various cellular processes with the decreased kinase activity of Cdk2. We hypothesize that pRb2/p130 may act not only to bind and modify E2F activity, but also to inhibit Cdk2 kinase activity in concert with p21 in a manner different from p107.