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 reg ulate G(1) progression and entry into the S phase of the cell cycle. I n mammalian cells, a key regulator of this process is the retinoblasto ma tumor suppressor protein (RE), In contrast, G(1) control in Sacchar omyces cerevisiae does not utilize an RE-like protein, We report here the cloning of cDNAs from two Zea mays genes, RRB1 and RRB2, that enco de RE-related proteins, Further, RRB2 transcripts are alternatively sp liced to yield two proteins with different C termini, At least one RRB gene is expressed in all the tissues examined, with the highest level s seen in the shoot apex, RRB1 is a 96-kDa nuclear protein that can ph ysically interact with two mammalian DNA tumor virus oncoproteins, sim ian virus 40 large-T antigen and adenovirus E1A, and with a plant D-ty pe 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 a nd B domains, although differences exist in these effects compared to those of human RE. RRB1 can also bind to the ALI protein from tomato g olden mosaic virus (TGMV), a protein which is essential for TGMV DNA r eplication, 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.