IDENTIFICATION AND ANALYSIS OF A RETINOBLASTOMA BINDING MOTIF IN THE REPLICATION PROTEIN OF A PLANT DNA VIRUS - REQUIREMENT FOR EFFICIENT VIRAL-DNA REPLICATION

Geminiviruses are plant DNA viruses with small genomes whose replicati on, except for the viral replication protein (Rep), depends on host pr oteins and, in this respect, are analogous to animal DNA tumor viruses , e.g. SV40. The mechanism by which these animal viruses create a cell ular environment permissive for viral DNA replication involves the bin ding of a virally encoded oncoprotein, through its LXCXE motif, to the retinoblastoma protein (Rb). We have identified such a LXCXE motif in the Rep protein of wheat dwarf geminivirus (WDV) and we show its func tional importance during viral DNA replication, Using a yeast two-hybr id system we have demonstrated that WDV Rep forms stable complexes wit h p130(Rbr2), a member of the Rb family of proteins, and single amino acid changes within the LXCXE motif abolish the ability of WDV Rep to bind to p130(Rbr2). The LXCXE motif is conserved in other members of t he same geminivirus subgroup, The presence of an intact Rb binding mot if is required for efficient WDV DNA replication in cultured wheat cel ls, strongly suggesting that one of the functions of WDV Rep may be th e linking between viral and cellular DNA replication cycles, Our resul ts point to the existence of a Rb-like protein(s) in plant cells playi ng regulatory roles during the cell cycle.