RRB1and RRB2Encode 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 G1progression 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 (RB). In contrast, G1control in Saccharomyces cerevisiaedoes not utilize an RB-like protein. We report here the cloning of cDNAs from two Zea maysgenes, RRB1and RRB2, that encode RB-related proteins. Further, RRB2transcripts are alternatively spliced to yield two proteins with different C termini. At least one RRBgene 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 RB 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 RB. RRB1 can also bind to the AL1 protein from tomato golden mosaic virus (TGMV), a protein which is essential for TGMV DNA replication. These results suggest that G1regulation in plant cells is controlled by a mechanism which is much more similar to that found in mammalian cells than that in yeast.