Tobacco RETINOBLASTOMA-RELATED protein is phosphorylated by different types of cyclin-dependent kinases during the cell cycle

Abstract

Plants contain a regulatory pathway similar to that involving retinoblastoma (RB) protein in animals. Here, we analyze an RB homolog, tobacco RETINOBLASTOMA-RELATED1 (NtRBR1), that encodes a protein possessing 13 potential phosphorylation sites for cyclin-dependent kinases (CDKs). We found that 11 synthetic peptides containing sequences from NtRBR1 were phosphorylated differentially by imunoprecipitates of cyclin/CDK complexes extracted from tobacco BY-2 cells. We raised antibodies that specifically recognize these sites of phosphorylation in NtRBR1. Distinct sites were phosphorylated during the cell cycle, suggesting that NtRBR1 is phosphorylated by different types of cyclin/CDK complexes.