RRB1 and RRB2 Encode Maize Retinoblastoma-Related Proteins That Interact with a Plant D-Type Cyclin and Geminivirus Replication Protein

Abstract

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 (RB). In contrast, G 1 control in Saccharomyces cerevisiae does not utilize an RB-like protein.We report here the cloning of cDNAs from two Zea mays genes, RRB1 and RRB2, that encode RB-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 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 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.Progression through the G 1 phase of the eukaryotic cell cycle is tightly regulated, allowing cells to integrate internal and external cues before initiating DNA replication and committing to a round of cell division. This process is governed by both positive-and negative-acting regulatory factors. Although substantial progress has been made in understanding the mechanisms that govern these events in yeast and mammals (reviewed in reference 61), relatively little is known about G 1 regulation in plants. The existence of cyclin-dependent kinases (Cdks) and their associated cyclin subunits in plants (reviewed in reference 14) suggests that at least some of the basic mechanisms which regulate the cell cycle have been conserved throughout eukaryotic evolution. However, identification of additional regulatory components of the plant cell cycle is clearly essential for understanding plant growth and development.In the yeast Saccharomyces cerevisiae, progression through the G 1 phase is regulated by the Cdk Cdc28 (50), which in conjunction with G 1 cyclins activates the heterodimeric Swi4/ Swi6 transcription factor (40), resulting in the transcriptional activation of genes necessary for G 1 progression and S-phase entry (12, 61). In mammalian cells, G 1 progression also depends upon a Cdk-cyclin-activated transcriptional control pathway. However, a major regulatory protein in this pathway, the retinoblastoma protein (RB), has not been found in yeast. RB is the 110-kDa product of the retinoblastoma susceptibility tumor suppressor gene and plays key roles in regulating both cell cycle progression through the G 1 ph...

Retinoblastoma family proteins as key targets of the small DNA virus oncoproteins

“…The alternating Leu, Cys and Glu residues point towards the inner part of the groove, making contacts with specific non-contiguous pRb residues. These residues are extremely conserved between pRb and pRb-like proteins of different species (Ach et al, 1997;Lee et al, 1998;Lu and Horvitz, 1998), indicating that this conserved groove of the B-box should play a critical role for the function of pRb, p107 and pRb2/p130.…”

ArabidopsisRETINOBLASTOMA-RELATED Is Required for Stem Cell Maintenance, Cell Differentiation, and Lateral Organ Production

“…The requirement for RB in cell differentiation therefore extends beyond regulation of cell cycle entry. For example, the loss of RB function in cardioblasts affects cell differentiation by modulating the activity of cardiogenic factors (Papadimou et al, 2005).RB-related genes, termed RETINOBLASTOMA-RELATED (RBR; Ach et al, 1997), are also found in monocots (Grafi et al, 1996;Xie et al, 1996;Ach et al, 1997) and dicots (Nakagami et al, 1999;Kong et al, 2000) and even in unicellular algae (Umen and Goodenough, 2001). Animal pRb and plant RBR are similar in sequence and predicted protein structure and, thus, likely share similar functions (Flemington et al, 1993;Ramirez-Parra et al, 2003;Gruissem, 2007).…”

Geminivirus Infection Up-Regulates the Expression of Two Arabidopsis Protein Kinases Related to Yeast SNF1- and Mammalian AMPK-Activating Kinases

“…AL1 and AL3 interact with the retinoblastoma-related protein (pRBR), a negative regulator of cell cycle progression and a differentiation factor (Weinberg, 1995;Ach et al, 1997;Kong et al, 2000). The AL1-pRBR interaction interferes with pRBR binding to the cell cycle-associated E2F transcription factors and leads to the activation of genes encoding proteins required for DNA replication (Egelkrout et al, 2001;Desvoyes et al, 2006).…”