Deregulation of cell proliferation is a hallmark of cancer. In many transformed cells, the cyclin A͞CDK2 complex that contains S-phase kinase associated proteins 1 and 2 (SKP1 and SKP2) is highly induced. To determine the roles of this complex in the cell cycle regulation and transformation, we have examined the composition of this complex. We report here that this complex contained an additional protein, human CUL-1, a member of the cullin͞CDC53 family. To ensure the faithful duplication and passage of genetic information during cell division, the transitions between different phases of the cell cycle are precisely coordinated and controlled by the cyclin-dependent kinases (CDKs) (1, 2). The sequential activation of each CDK in the cell cycle primarily is achieved by its association with a specific regulatory subunit, a cyclin. In mammals, the G 1 cell cycle progression is controlled by the activity of cyclin D͞CDK4 or CDK6, which specifically phosphorylates and thus inactivates the growth suppression activity of retinoblastoma susceptibility gene product, pRb (2). In late G 1 , both cyclin E͞CDK2 and cyclin A͞CDK2 play a role during the G 1 ͞S transition, although their critical targets are mostly undefined.The cyclin͞CDK activities are further regulated by both positive and negative phosphorylation and the association of CDK inhibitors such as p21 CIP1/WAF1 , p27 KIP1 , and p16 INK4A(1, 2). The CDK inhibitors usually act as checkpoint control proteins to prevent the unscheduled entry of the S phase. p21 has been shown to be regulated by p53 tumor suppressor during the DNA damage response (2). p16, a specific CDK4 or CDK6 inhibitor, is itself encoded by a tumor susceptibility gene located on the chromosome 9p21 locus, the loss of which is associated with a wide variety of human cancer, including familial melanomas (2).Previously we found that in normal human fibroblasts, a substantial fraction of cyclin A͞CDK2 was associated with p21 and the proliferating cell nuclear antigen (PCNA) (3). However, in many DNA viral oncoprotein transformed or other established tumor cells that are deficient in p53 expression, p21 and PCNA disappeared and cyclin A͞CDK2 was prominently complexed with two novel proteins, S-phase kinase associated proteins 1 and 2 (SKP1 and SKP2) (4). To investigate the significance of this change, we have isolated the p19 SKP1 ͞p45 SKP2 ͞cyclin A͞CDK2 complex and cloned the genes encoding p19 SKP1 and p45 SKP2 (4). Our studies indicate that p45 SKP2 expression is highly induced in many transformed cells (4). In addition, ablation of p45 SKP2 by antibody microinjection into G 1 cells prevented these cells from entry into the S phase, suggesting that p45 SKP2 and probably p19 SKP1 , is required for the G 1 ͞S transition (4). p45 SKP2 contains a leucine-rich-repeated domain at its carboxyl terminus and a small, but relatively conserved, motif, an F-box, at its amino terminal region, which was later found to mediate the interaction with p19 SKP1 (5).Recently, a yeast SKP1 homologue was isola...

Activation of the p53-mediated DNA damage response induces either G 1 cell cycle arrest or apoptosis. The G 1 cell cycle arrest is in part caused by the p53-dependent transcriptional activation of the CDK inhibitor, p21 Cip1/ Waf1. We report here that human p21 protein is rapidly induced but selectively cleaved during the apoptotic response to ␥-irradiation. Such an event occurred early, well before the morphological appearance of apoptosis. Ectopical expression of p53 in tumor cells alone could induce p21 expression, followed by p21 cleavage and apoptosis. The cleavage of p21 could be reproduced in extracts prepared from irradiated cells or by recombinant caspase-3, suggesting that a caspase-like activity is responsible for this cleavage. p21 binds independently to both CDK2 and proliferation cell nuclear antigen (PCNA). Our studies indicated that p21 cleavage by the caspase-like activity specifically abolished its interaction with PCNA, suggesting that p21 cleavage may interfere with normal PCNA-dependent repair. Our data suggest that p21 may serve as a critical checkpoint regulator for both cell cycle arrest and apoptosis during the p53-mediated DNA damage response. Manipulation of the checkpoint regulators involved in cell cycle arrest and apoptosis may thus provide a novel strategy to cancer therapy.

De nombreux travaux ont été consacrés à la caractérisation des niveaux pièges introduits par l'or ou le platine dans le silicium. Pourtant les paramètres caractéristiques de ces niveaux ne sont connus qu'avec une assez grande dispersion surtout en ce qui concerne le platine dans le silicium. Les résultats présentés ici ont été obtenus sur des jonctions P+ N graduelles dont la densité d'impuretés Au ou Pt est du même ordre de grandeur que celle des donneurs. Plusieurs méthodes de caractérisation des niveaux piègescapacité en régime transitoire, D.L.T.S., T.C.T.S., spectroscopie d'admittanceont été mises en oeuvre afin de déterminer les taux d'émission thermique de chaque niveau sur le plus grand intervalle de variations possible (10-2 s-1 à 107 s-1), condition pour laquelle les niveaux d'énergie et surtout les sections efficaces de capture peuvent être donnés avec le maximum de précision. Les résultats confirment que l'or introduit dans le silicium un niveau accepteur de signature en(T0/T)2 = 2,5 x 1012 exp(-0,546/kT) et un niveau donneur de signature ep(T0/T)2 = 5,1 x 1013 exp(-0,360/kT). D'autre part le platine introduit aussi dans le silicium deux niveaux principaux, un niveau accepteur de signature en(T0/T)2 = 1013 exp(-0,230/kT), et un niveau donneur de signature ep(T0/T)2 = 6 x 1012 exp(-0,32/kT). Ce dernier niveau, qui est toujours donné avec une très grande imprécision, pourraît être constitué de deux niveaux très rapprochés compris entre 0,30 eV et 0,36 eV. Dans le cas de surcompensation et lorsque le platine est diffusé dans une base de silicium dopé au phosphore, il apparait un niveau de très faible densité situé à Ev + 0,42 eV. Abstract.-Many works have been devoted to the characterisation of trap levels introduced by gold and platinum impurities in silicon. However the parameters of these levels are not well defined specially those assigned to platinum.