Retinoblastoma-related proteins in lower eukaryotes

Abstract

Retinoblastoma-related proteins (RBRs) are key regulators and hubs in a complex regulatory network that controls cell cycle and cell division. RBRs have also been shown to play a role in the regulation of a wide variety of other cellular processes including tumor suppression, DNA repair, DNA damage checkpoint control, differentiation, cellular senescence, and apoptosis. The factors directly interacting with RBRs are cyclins, kinases, transcription factors and chromatin remodeling complexes. The retinoblastoma tumor suppressor pathway has been investigated in detail in higher organisms as it was assumed that RBRs only have a widespread role in advanced multicellular development. This review gives an overview of the phylogenetic distribution of RBRs and particularly emphasizes the presence of RBRs in lower eukaryotes. RBRs were not only identified in animals (Metazoa) but also in Viridiplantae (Monocots, Dicots, Lycophytes, mosses and green algae), Heteroconta (Oomycetes, diatoms and brown algae), Alveolata, red algae (Rhodophyta), Heterolobosea, Haptophyta, and Amoebozoa. Only the Fungi seem to be an exception, because only the most primitive phylum retained an RBR, while the more advanced phyla evolved a functionally similar key regulator instead. RBRs are clearly not a particular characteristic of multicellular eukaryotes, as they are also common to unicellular eukaryotes. Thus, RBRs are highly conserved and much more widespread among eukaryotes than previously thought.