Activation through TCR/CD3-plus-CD28 induces primary T lymphocytes to enter Sphase. Downregulation of cyclin-dependent kinase inhibitor p27kip1 is critical in thisprocess and is mediated by ubiquitin-targeted degradation of p27kip1. Ubiquitination ofp27kip1 is performed by the SCFskp2 ubiquitin ligase comprised of the core componentsRoc1, Cul1 and Skp1 and the substrate recognition components Skp2 and Cks1. Here weshow that in primary human T lymphocytes, the SCFskp2 core components Roc1, Cul1and Skp1 are constitutively expressed and their levels remain unchanged uponTCR/CD3-plus-CD28 costimulation. In contrast, the substrate recognition componentsSkp2 and Cks1 are almost undetectable and are transcriptionally induced uponcostimulation. We determined that the SKP2 promoter lies directly upstream of thetranscription start site and contains binding sites for SP1, Elk-1 and E2F transcriptionfactors. Mutagenesis of SP1 and Elk-1 sites abrogated TCR/CD3-plus-CD28-mediatedSKP2 promoter-driven reporter activity, whereas mutagenesis of E2F site enhancedreporter activity, suggesting that SKP2 promoter may act as a node of integration formitogenic and anti-mitogenic signals. Thus, in primary T lymphocytes CD28costimulation can directly regulate cell cycle progression by inducing transcription of thesubstrate recognition components of SCFskp2 ubiquitin ligase that targets p27kip1 fordegradation.