The two distinct proteins encoded by the CDKN2A locus are specified by translating the common second exon in alternative reading frames. The product of the α transcript, p16 INK4a , is a recognized tumour suppressor that induces a G 1 cell cycle arrest by inhibiting the phosphorylation of the retinoblastoma protein by the cyclin-dependent kinases, CDK4 and CDK6. In contrast, the product of the human CDKN2A β transcript, p14 ARF , activates a p53 response manifest in elevated levels of MDM2 and p21 CIP1 and cell cycle arrest in both G 1 and G 2 /M. As a consequence, p14 ARFinduced cell cycle arrest is p53 dependent and can be abrogated by the co-expression of human papilloma virus E6 protein. p14 ARF acts by binding directly to MDM2, resulting in the stabilization of both p53 and MDM2. Conversely, p53 negatively regulates p14 ARF expression and there is an inverse correlation between p14 ARF expression and p53 function in human tumour cell lines. However, p14 ARF expression is not involved in the response to DNA damage. These results place p14 ARF in an independent pathway upstream of p53 and imply that CDKN2A encodes two proteins that are involved in tumour suppression.

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To investigate the mode of action of the p16INK4a tumor suppressor protein, we have established U2-OS cells in which the expression of p16INK4a can be regulated by addition or removal of isopropyl-␤-D-thiogalactopyranoside. As expected, induction of p16INK4a results in a G 1 cell cycle arrest by inhibiting phosphorylation of the retinoblastoma protein (pRb) by the cyclin-dependent kinases CDK4 and CDK6. However, induction of p16INK4a also causes marked inhibition of CDK2 activity. In the case of cyclin E-CDK2, this is brought about by reassortment of cyclin, CDK, and CDK-inhibitor complexes, particularly those involving p27 KIP1 . Size fractionation of the cellular lysates reveals that a substantial proportion of CDK4 participates in active kinase complexes of around 200 kDa. Upon induction of p16INK4a , this complex is partly dissociated, and the majority of CDK4 is found in lower-molecular-weight fractions consistent with the formation of a binary complex with p16INK4a . Sequestration of CDK4 by p16 INK4a allows cyclin D1 to associate increasingly with CDK2, without affecting its interactions with the CIP/KIP inhibitors. Thus, upon the induction of p16 INK4a, p27KIP1 appears to switch its allegiance from CDK4 to CDK2, and the accompanying reassortment of components leads to the inhibition of cyclin E-CDK2 by p27 KIP1 and p21 CIP1 . Significantly, p16 INK4a itself does not appear to form higher-order complexes, and the overwhelming majority remains either free or forms binary associations with CDK4 and CDK6.

Structure of the chromatin binding (chromo) domain from mouse modifier protein 1 protein, an activator, might also be localized to chromatin Linda J.Ball, Natalia V.Murzina, via its chromo domain (Koonin et al., 1995).

The ARF protein, encoded by alternate exon usage within the CDKN2A locus, provides a link between the retinoblastoma (pRb) and p53 tumor suppressor pathways. Agents that disable pRb or otherwise impinge on the E2F family of transcription factors induce expression of ARF, resulting in stabilization of p53 and activation of p53-regulated genes. However, in some cell types ARF is not induced upon cell cycle re-entry, as expected of a conventional E2F target gene, leading to the suggestion that the ARF promoter only responds to supra-physiological or aberrant levels of E2F. These properties have recently been attributed to a variant E2F binding site but attempts to map specific response elements within the ARF promoter have generally yielded confusing answers. Here we show that in IL2-dependent T-lymphocytes, ARF expression is induced as cells progress from G 0 into S phase, in parallel with other bona fide E2F target genes. This is accompanied by increased association of E2F1 with the endogenous ARF promoter. Our findings suggest that the ability of ARF to register normal proliferative cues depends on the levels of E2F generated in different settings and argue against the idea that it reacts exclusively to oncogenic signals.