A maize cDNA encoding a member of the retinoblastoma protein family: Involvement in endoreduplication

Retinoblastoma (RB-1) is a tumor suppressor gene that encodes a 105-kDa nuclear phosphoprotein, To date, RE genes have been isolated only from metazoans. We have isolated a cDNA from maize endosperm whose predicted protein product (ZmRb) shows homology to the ''pocket'' A and B domains of the Rb protein family, We found ZmRb behaves as a pocket protein based on its ability to specifically interact with oncoproteins encoded by DNA tumor viruses (E7, T-Ag, E1A), ZmRb can interact in vitro and in vivo with the replication-associated protein, RepA, encoded by the wheat dwarf virus, The maize Rb-related protein undergoes changes in level and phosphorylation state concomitant with endoreduplication, and it is phosphorylated in vitro by an S-phase kinase from endoreduplicating endosperm cells, Together, our results suggest that ZmRb is a representative of the pocket protein family and may play a role in cell cycle progression, Moreover, certain plant monopartite geminiviruses may operate similarly to mammalian DNA viruses, by targeting and inactivating the retinoblastoma protein, which otherwise induces G(1) arrest.