ReviewConserved functions of retinoblastoma proteins: From purple retina to green plant cells
Introduction
The retinoblastoma susceptibility gene was the firstly identified tumor suppressor and was isolated by positional cloning from retinoblastoma tumors [1] that originated from the thin membranous retina situated in the back of the eye. Since then, many investigations showed a broad range of functions for the retinoblastoma protein (pRB) in higher eukaryotic organisms. The retinoblastoma gene family encodes a group of related proteins that participate in several processes of cell growth and differentiation, including cell cycle regulation and control of gene expression [2], [3], [4]. It was shown more than a decade ago, that many of the fundamental control mechanisms that govern cell division in animals are also conserved in plants [5], [6], [7], [8], [9]. The first retinoblastoma homologue gene of plant origin was cloned from Zea mays [10], [11], [12]; subsequently similar genes have been isolated from several other plant species, including the dicot Nicotiana tabacum [13], Arabidopsis thaliana [14], Medicago sativa and the monocot Oryza sativa [15].
Considering the phylogenetic conservation of several cell cycle regulatory elements (CDKs; cyclins) between animals and plants, one might predict that the retinoblastoma-related plant homologues play a complex role, analogous to many functions found in their animal counterparts. However, the plant life cycle possesses several specific features compared to animals, for example, plant development is largely postembryonic; cell divisions occur in specialized zones known as meristems; somatic cells are totipotent. Therefore, we can expect cell cycle regulatory elements in plants – including RB proteins – to show a plant specific nature with respect to growth and developmental control.
Section snippets
Conserved structure of the RB proteins
In order to evaluate the functional significance of RBs in control mechanisms, it is worth discussing some of the structural features of these proteins.
Since our present knowledge of the structural and functional characteristics of the RB protein family is derived mainly from the original RB protein itself, we use this 928 amino acid long human RB protein to present the significant functional regions, as summarized in Fig. 1.
The amino-terminal region of pRB (amino acids 1–378) is crucial for
Broad functions of the retinoblastoma protein family in animals
In the middle of the past century the children's retinoblastoma tumor was realized not only to occur sporadically, but also to be inherited [46]. Cloning of the retinoblastoma susceptibility gene [47] and identification of its mutations [48] opened an innovative chapter in cancer molecular biology. Soon, further retinoblastoma homologue genes were isolated, and ever since they have been known as a family of pocket proteins, consisting of three members, pRB, p107 [49] and p130 [50] with
Retinoblastoma-related genes from plants
A partial cDNA clone of the first identified plant retinoblastoma-related gene was isolated from Z. mays [10]. Subsequently, another cDNA was also isolated, which was considered to be a partial coding sequence for ZmRBR (ZmRBR2), the encoded protein was approximately 90% identical to ZmRBR1[12]. The predicted protein products of the open reading frames showed an analogous domain structure to the animal RB family and showed a persuading similarity to the A and B pocket region [10], [11], [12].
Final conclusions and prospects
The complexity of RBR functions in higher plants can be concluded from the wide variety of roles of RBR proteins discussed before [67], [96], [110], [131]. It is assumptive that the evolution of the RB pathway occurred after separation of the fungi from the higher eukaryotic lineage, but preceded the divergence of the plant and animal kingdoms [75]. While animals have at least two homologues of pocket domain proteins, it is an interesting finding that the monocot Gramineae species similarly
Acknowledgments
We would like to thank Mátyás Cserháti for critical reading of the manuscript. Gábor V. Horváth is grateful for the support of the “János Bólyai” Research Fellowship.
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