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-kD a nuclear phosphoprotein, To date, RE genes have been isolated only fr om metazoans. We have isolated a cDNA from maize endosperm whose predi cted protein product (ZmRb) shows homology to the ''pocket'' A and B d omains of the Rb protein family, We found ZmRb behaves as a pocket pro tein based on its ability to specifically interact with oncoproteins e ncoded by DNA tumor viruses (E7, T-Ag, E1A), ZmRb can interact in vitr o and in vivo with the replication-associated protein, RepA, encoded b y the wheat dwarf virus, The maize Rb-related protein undergoes change s in level and phosphorylation state concomitant with endoreduplicatio n, and it is phosphorylated in vitro by an S-phase kinase from endored uplicating 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 geminiviru ses may operate similarly to mammalian DNA viruses, by targeting and i nactivating the retinoblastoma protein, which otherwise induces G(1) a rrest.