Structure-function analysis of the retinoblastoma tumor suppressor protein - is the whole a sum of its parts?

p107 in the public eye: an Rb understudy and more

p107 and its related family members Rb and p130 are critical regulators of cellular proliferation and tumorigenesis. Due to the extent of functional overlap within the Rb family, it has been difficult to assess which functions are exclusive to individual members and which are shared. Like its family members, p107 can bind a variety of cellular proteins to affect the expression of many target genes during cell cycle progression. Unlike Rb and p130, p107 is most highly expressed during the G1 to S phase transition of the cell cycle in actively dividing cells and accumulating evidence suggests a role for p107 during DNA replication. The specific roles for p107 during differentiation and development are less clear, although emerging studies suggest that it can cooperate with other Rb family members to control differentiation in multiple cell lineages. As a tumor suppressor, p107 is not as potent as Rb, yet studies in knockout mice have revealed some tumor suppressor functions in mice, depending on the context. In this review, we identify the unique and overlapping functions of p107 during the cell cycle, differentiation, and tumorigenesis.

A cancer derived mutation in the retinoblastoma gene with a distinct defect for LXCXE dependent interactions

BACKGROUND

The interaction between viral oncoproteins such as Simian virus 40 TAg, adenovirus E1A, and human papilloma virus E7, and the retinoblastoma protein (pRB) occurs through a well characterized peptide sequence, LXCXE, on the viral protein and a well conserved groove in the pocket domain of pRB. Cellular proteins, such as histone deacetylases, also use this mechanism to interact with the retinoblastoma protein to repress transcription at cell cycle regulated genes. For these reasons this region of the pRB pocket domain is thought to play a critical role in growth suppression.

RESULTS

In this study, we identify and characterize a tumor derived allele of the retinoblastoma gene (RB1) that possesses a discrete defect in its ability to interact with LXCXE motif containing proteins that compromises proliferative control. To assess the frequency of similar mutations in the RB1 gene in human cancer, we screened blood and tumor samples for similar alleles. We screened almost 700 samples and did not detect additional mutations, indicating that this class of mutation is rare.

CONCLUSIONS

Our work provides proof of principal that alleles encoding distinct, partial loss of function mutations in the retinoblastoma gene that specifically lose LXCXE dependent interactions, are found in human cancer.

Targeting mechanism of the retinoblastoma tumor suppressor by a prototypical viral oncoprotein. Structural modularity, intrinsic disorder and phosphorylation of human papillomavirus E7

DNA tumor viruses ensure genome amplification by hijacking the cellular replication machinery and forcing infected cells to enter the S phase. The retinoblastoma (Rb) protein controls the G1/S checkpoint, and is targeted by several viral oncoproteins, among these the E7 protein from human papillomaviruses (HPVs). A quantitative investigation of the interaction mechanism between the HPV16 E7 protein and the RbAB domain in solution revealed that 90% of the binding energy is determined by the LxCxE motif, with an additional binding determinant (1.0 kcal.mol(-1)) located in the C-terminal domain of E7, establishing a dual-contact mode. The stoichiometry and subnanomolar affinity of E7 indicated that it can bind RbAB as a monomer. The low-risk HPV11 E7 protein bound 2.0 kcal.mol(-1) more weakly than the high-risk HPV16 and HPV18 type counterparts, but the modularity and binding mode were conserved. Phosphorylation at a conserved casein kinase II site in the natively unfolded N-terminal domain of E7 affected the local conformation by increasing the polyproline II content and stabilizing an extended conformation, which allowed for a tighter interaction with the Rb protein. Thus, the E7-RbAB interaction involves multiple motifs within the N-terminal domain of E7 and at least two conserved interaction surfaces in RbAB. We discussed a mechanistic model of the interaction of the Rb protein with a viral target in solution, integrated with structural data and the analysis of other cellular and viral proteins, which provided information about the balance of interactions involving the Rb protein and how these determine the progression into either the normal cell cycle or transformation.

Regulation of the retinoblastoma proteins by the human herpesviruses

Viruses are obligate intracellular parasites that alter the environment of infected cells in order to replicate more efficiently. One way viruses achieve this is by modulating cell cycle progression. The main regulators of progression out of G0, through G1, and into S phase are the members of the retinoblastoma (Rb) family of tumor suppressors. Rb proteins repress the transcription of genes controlled by the E2F transcription factors. Because the expression of E2F-responsive genes is required for cell cycle progression into the S phase, Rb arrests the cell cycle in G0/G1. A number of viral proteins directly target Rb family members for inactivation, presumably to create an environment more hospitable for viral replication. Such viral proteins include the extensively studied oncoproteins E7 (from human papillomavirus), E1A (from adenovirus), and the large T (tumor) antigen (from simian virus 40). Elucidating how these three viral proteins target and inactivate Rb has proven to be an invaluable approach to augment our understanding of both normal cell cycle progression and carcinogenesis. In addition to these proteins, a number of other virally-encoded inactivators of the Rb family have subsequently been identified including a surprising number encoded by human herpesviruses. Here we review how the human herpesviruses modulate Rb function during infection, introduce the individual viral proteins that directly or indirectly target Rb, and speculate about what roles Rb modulation by these proteins may play in viral replication, pathogenesis, and oncogenesis.

Conserved functions of the pRB and E2F families

Proteins that are related to the retinoblastoma tumour suppressor pRB and the E2F transcription factor are conserved in many species of plants and animals. The mammalian orthologues of pRB and E2F are best known for their roles in cell proliferation, but it has become clear that they affect many biological processes. Here we describe the functions of pRB-related proteins and E2F proteins that have emerged from genetic and biochemical experiments in Caenorhabditis elegans and Drosophila melanogaster. The similarities that have been observed between worms, flies and mammals provide insight into the core activities of pRB and E2F proteins and show how a common regulatory module can control various biological functions in different organisms.

A gender-specific retinoblastoma-related protein in Volvox carteri implies a role for the retinoblastoma protein family in sexual development

Here, we describe the cloning and characterization of RETINOBLASTOMA-RELATED PROTEIN1 (RBR1) from the green alga Volvox carteri. RBR1 expression increases substantially during embryogenesis and in response to the sex-inducer glycoprotein, but it decreases significantly under heat stress. While RBR1 is expressed in gonidia (asexual reproductive cells) and embryos, the largest proportion of RBR1 mRNA is found in parental somatic cells. The presence of 4 splice variants and 15 potential cyclin-dependent kinase phosphorylation sites suggests that RBR1 is subject to control at the posttranscriptional and posttranslational levels. Surprisingly, RBR1 is a gender-specific gene, mapping exclusively to the female mating-type locus. A procedure for stable nuclear transformation of males was established to generate RBR1-expressing males. These transformants exhibit enlarged reproductive cells, altered growth characteristics, and a prolonged embryogenesis. The results suggest that a functionally related analog of RBR1 exists in males. The reason for the divergent evolution of RBRs in females and males appears to be based on sexual development: males and females respond to the same sex-inducer with different cleavage programs and substantial differences in cellular differentiation. Thus, the gender-specific presence of RBR1 provides evidence for an additional, novel role for retinoblastoma family proteins in sexual development.

Cellular mechanisms of tumour suppression by the retinoblastoma gene

The retinoblastoma (RB) tumour suppressor gene is functionally inactivated in a broad range of paediatric and adult cancers, and a plethora of cellular functions and partners have been identified for the RB protein. Data from human tumours and studies from mouse models indicate that loss of RB function contributes to both cancer initiation and progression. However, we still do not know the identity of the cell types in which RB normally prevents cancer initiation in vivo, and the specific functions of RB that suppress distinct aspects of the tumorigenic process are poorly understood.

A locking mechanism regulates RNA synthesis and host protein interaction by the hepatitis C virus polymerase

Mutational analysis of the hepatitis C virus (HCV) RNA-dependent RNA polymerase (RdRp) template channel identified two residues, Trp(397) and His(428), which are required for de novo initiation but not for extension from a primer. These two residues interact with the Delta1 loop on the surface of the RdRp. A deletion within the Delta1 loop also resulted in comparable activities. The mutant proteins exhibit increased double-stranded RNA binding compared with the wild type, suggesting that the Delta1 loop serves as a flexible locking mechanism to regulate the conformations needed for de novo initiation and for elongative RNA synthesis. A similar locking motif can be found in other viral RdRps. Products associated with the open conformation of the HCV RdRp were inhibited by interaction with the retinoblastoma protein but not cyclophilin A. Different conformations of the HCV RdRp can thus affect RNA synthesis and interaction with cellular proteins.

[Regenerative medicine in the treatment of sensorineural hearing loss]

Regenerative medicine offers the prospect of causal treatment of sensorineural hearing loss. In humans, the loss of sensory hair cells is irreversible and results in chronic hearing loss. Other vertebrates, particularly birds, have the capability to spontaneously regenerate lost sensory hair cells and restore hearing. In the bird model, regeneration of hair cells is based on the proliferation of supporting cells. In mammals, supporting cells have lost their proliferative capacity and are terminally differentiated. To gain an understanding about regeneration of hair cells in mammals, cell division of supporting cells has to be controlled. Gene disruption of the cell cycle inhibitor p27(Kip1) allows supporting cell proliferation in the organ of Corti in vivo. Furthermore, in vitro studies indicate that newly generated cells may differentiate into hair cells after p27(Kip1) disruption. Other current methods to induce hair cell regeneration include the gene transfer of Math1 and transplantation of stem cells to the inner ear.