The ARF tumour suppressor interferes with the MDM2 E3 ubiquitin protein ligase to stabilize and activate the p53 transcription factor, triggering cell-cycle arrest or apoptosis.
It is widely accepted that the activity of ARF is mediated through the activation of the p53 transcription programme, but several lines of evidence, much of it controversial, indicate that ARF also exerts p53-independent tumour-suppressor functions.
The ARF protein has an unusual amino-acid composition, being highly basic (pI>12, despite a paucity of lysine residues); it is probably unstructured unless bound to other targets and highly promiscuous in its binding.
ARF is a nucleolar protein that assembles into high-molecular-mass complexes with nucleophosmin (NPM). Its binding to NPM inhibits ARF turnover and results in its accumulation within the nucleolus.
ARF has an associated sumoylating activity that can lead to the modification of proteins to which it binds, including MDM2 and NPM.
ARF has now been reported to physically interact with more than 25 other proteins, at least some of which have been postulated to be responsible for its p53-independent functions. These include proteins involved in ribosome biogenesis, transcriptional regulation, the DNA-damage response, apoptosis and autophagy. How strong are the data?
Mammalian cells that sustain oncogenic insults can invoke defensive programmes that either halt their division or trigger their apoptosis, but these countermeasures must be finely tuned to discriminate between physiological and potentially harmful growth-promoting states. By functioning specifically to oppose abnormally prolonged and sustained proliferative signals produced by activated oncogenes, the ARF tumour suppressor antagonizes functions of MDM2 to induce protective responses that depend on the p53 transcription factor and its many target genes. However, ARF has been reported to physically associate with proteins other than MDM2 and to have p53-independent activities, most of which remain controversial and poorly understood.
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The author thanks M. B. Kastan, J. L. Cleveland, J. T. Opferman and M. F. Roussel for critical comments and suggestions.
The author declares no competing financial interests.
- Retinoblastoma protein
(RB) Named for its tumour-suppressor role in familial retinoblastoma, RB and its related family members (p130 and p107) function as transcriptional co-repressors to prevent cells from entering the DNA synthesis (S) phase of the cell division cycle.
- Cyclin-dependent kinases
(CDKs) These enzymes are composed of a catalytic CDK subunit and a regulatory cyclin subunit. The cyclin D-dependent CDKs are activated in response to extracellular mitogenic signals and phosphorylate the retinoblastoma protein (RB) and other RB-family members (p107 and p130).
- E3 ubiquitin ligase
The addition of polyubiquitin chains to proteins is carried out by a cascade of three enzymes, designated E1, E2 and E3, that sequentially activate and transfer ubiquitin. The selection of protein substrates for ubiquitylation is mediated by the E3 enzymes.
- Hyaloid vascular system
(HVS) During eye development, this delicate vasculature extends from the optic cup in the back of the eye through the vitreous to supply the lens. In the mouse, the HVS involutes within the first 10 days postnatally, correlating with maximal p19ARF expression in pericytes surrounding the vascular endothelium.
- INK4 proteins
So named because they are inhibitors of CDK4, these proteins associate with and block the enzymatic activities of the cyclin D-dependent kinases CDK4 and CDK6. There are four such proteins, designated INK4a, INK4b, INK4c, and INK4d in order of their discovery.
- Tandem-affinity tagging
The synthesis of a recombinant protein engineered to include multiple 'tags' (for example, small peptides added to its N or C terminus) enables it to be biochemically recovered through sequential affinity purification steps usually performed using antibodies to the tags.
- Isoelectric point
The pH at which dipolar molecules do not migrate in an electrical field. Neutralization of the charge of the highly arginine-rich ARF protein requires a pH >12.
The small ubiquitin-like modifier (SUMO) is conjugated to target proteins in a manner analogous to ubiquitylation. The biological consequences of sumoylation are relatively poorly understood, although the process might regulate diverse cellular activities such as protein trafficking, chromatin remodelling and gene expression.
An intranuclear organelle that is the primary site of ribosomal DNA transcription, rRNA processing, ribosome assembly and transport.
A macromolecular 'machine' that hydrolyses proteins into their constituent amino acids.
MYC is a transcription factor that, following heterodimerization with its partner MAX, can bind directly to canonical hexameric DNA sequences found in the promoters of many genes. Depending on other proteins recruited to promoters, the MYC–MAX complex can either transactivate or repress gene expression.
- Radius of gyration
A parameter characterizing the size of a particle of any shape, used in this context to refer to different penultimate amino acids at the N terminus of a polypeptide chain.
- ATR and ATM
ATM, the gene mutated in the ataxia-telangiectasia syndrome, encodes a protein kinase whose activity is triggered by DNA double-strand breaks. The ATM and RAD3-related kinase, ATR, has a similar role but is primarily activated by stalled replication forks.
- Nuclear factor κB
A family of transcription factors that regulate the inflammatory response. Some NFκB family members interfere with apoptosis.
A modified histone H2 variant, the phosphorylation of which occurs at chromosomal sites of DNA damage.
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Sherr, C. Divorcing ARF and p53: an unsettled case. Nat Rev Cancer 6, 663–673 (2006). https://doi.org/10.1038/nrc1954
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