RRB1 and RRB2 encode maize retinoblastoma-related proteins that interact with a plant D-type cyclin and geminivirus replication protein

Unlike mammalian and yeast cells, little is known about how plants regulate G(1) progression and entry into the S phase of the cell cycle. In mammalian cells, a key regulator of this process is the retinoblastoma tumor suppressor protein (RE), In contrast, G(1) control in Saccharomyces cerevisiae does not utilize an RE-like protein, We report here the cloning of cDNAs from two Zea mays genes, RRB1 and RRB2, that encode RE-related proteins, Further, RRB2 transcripts are alternatively spliced to yield two proteins with different C termini, At least one RRB gene is expressed in all the tissues examined, with the highest levels seen in the shoot apex, RRB1 is a 96-kDa nuclear protein that can physically interact with two mammalian DNA tumor virus oncoproteins, simian virus 40 large-T antigen and adenovirus E1A, and with a plant D-type cyclin, These associations are abolished by mutation of a conserved cysteine residue in RRB1 that is also essential for KB function, RRB1 binding potential is also sensitive to deletions in the conserved A and B domains, although differences exist in these effects compared to those of human RE. RRB1 can also bind to the ALI protein from tomato golden mosaic virus (TGMV), a protein which is essential for TGMV DNA replication, These results suggest that G(1) regulation in plant cells is controlled by a mechanism which is much more similar to that found in mammalian cells than that in yeast.