Modification of the plant retinoblastoma-related protein(RBR1) by SUMO : structural and functional studies

Abstract: The retinoblastoma protein (Rb) is a transcription regulator and key component of theRb/E2F/DP pathway which regulates progression of the cell cycle in plants and animals.Within the pathway, Rb blocks E2F transcriptional activity consequently ensuringrestricted cell proliferation. Of great importance too, is a family of posttranslationalmodifiers referred to as small ubiquitin-related modifiers (SUMO), whose modificationconsequences include; sub cellular localization of proteins, alteration of protein to proteininteraction and regulation of transcriptional activity.In order to study and depict the plant retinoblastoma related protein (RBR1) as a SUMOsubstrate; its modification site was mutated to address the effect of the mutation onprotein localization. Additionally, an in-vitro assay was used to further illustrate theconsequences of the mutation. In protoplasts transfected with wild type RBR1 the proteinwas solely present in the nucleus while those transfected with mutated RBR1, the proteinwas seen in both the nucleus and the cytosol. From the in vitro SUMOylation assay it wasevident that while wild type RBR1 could be modified by SUMO, its mutated versioncould not undergo modification.The results from this study don’t only show RBR1 as a SUMO substrate; they alsosuggest that modification by SUMO could be needed for its sub-cellular localization.