The D-type cyclin-dependent kinases CDK4 and CDK6 are complexed with many small cellular proteins (p14, plS, p16, plS, and p20). We have isolated cDNA sequences corresponding to the MTS2 genomic fragment that encodes the CDK4-and CDK6-associated p14 protein. By use of a yeast interaction screen to search for CDK6-interacting proteins, we have also identified an 18-kD human protein, p18, that is a homolog of the cyclin D-CDK4 inhibitors p16 {INK4A/MTS1) and p14 (MTS2/INK4B). Both in vivo and in vitro, p18 interacts strongly with CDK6, weakly with CDK4, and exhibits no detectable interaction with the other known CDKs. Recombinant p18 inhibits the kinase activity of cyclin D-CDK6. Distinct from the p21/p27 family of CDK inhibitors that form ternary complexes with cyclin-CDKs, only binary complexes of p14, p16, and p18 were found in association with CDK4 and/or CDK6. Ectopic expression of p18 or p16 suppresses cell growth with a correlated dependence on endogenous wild-type pRb.[Key Words: Cyclin-dependent kinase inhibitors; cell cycle; CDK4 and CDK6 interacting proteins]

Transforming growth factor ␤ (TGF-␤) causes growth arrest in the G 1 phase in many cell types. One probable pathway for this growth inhibition is through the TGF-␤-mediated up-regulation of the cyclin-dependent kinase (CDK) inhibitor p15 INK4B, which specifically inhibits the enzymatic activities of CDK4 and CDK6. An active cyclin D-CDK4/6 complex is required for pRb phosphorylation to allow the cell cycle to progress from G 1 to S phase. To study the molecular mechanism of the p15 INK4B induction by TGF-␤, we isolated a 780-base pair promoter sequence of the human p15 gene and inserted this fragment upstream of a luciferase reporter gene. When this construct was transiently transfected into HaCaT cells, luciferase activity was induced more than 10-fold upon TGF-␤ treatment, indicating that the induction of p15 INK4B expression by TGF-␤ is partly exerted at the transcription level. Promoter deletion analysis revealed that the sequence from ؊110 to ؊40 relative to the transcription start site is capable of conferring the 10-fold induction by TGF-␤. Within this region there are three Sp1 consensus sites. Mutation of one of these sites, GGGGCGGAG, substantially reduced both the induction by TGF-␤ and the basal promoter activity, whereas mutations in the other two Sp1 sites and the spacer sequences had little effect. In addition, gel mobility shift assay indicates that the transcription factors Sp1 and Sp3 bind to this Sp1 site. Taken together, these data suggest that a specific Sp1 consensus site is involved in the mediation of TGF-␤ induction as well as the basal promoter activity of the p15 gene and that Sp1 and Sp3 transcription factors might be involved in this regulation.Transforming growth factor ␤s (TGF-␤s) 1 represent a large family of cytokines with diverse activities in the regulation of cell growth, differentiation, and morphogenesis (1-3). TGF-␤ causes growth inhibition of most epithelial, endothelial, fibroblast, neuronal, lymphoid, and hematopoietic cell types (3). TGF-␤ treatment induces growth arrest in the G 1 phase of the cell cycle, and this effect has been attributed largely to an inhibition of phosphorylation of the retinoblastoma susceptibility gene product, pRb (4). Progression through the G 1 phase of the cell cycle requires phosphorylation of pRb by G 1 cyclin-dependent kinase (CDK) complexes, particularly the cyclin D-CDK4 and cyclin D-CDK6 complexes (5). Phosphorylation of pRb releases transcription factors, including members of the E2F transcription factor family, required for the G 1 to S phase transition of the cell cycle (6).Two distinct families of CDK inhibitors, represented by p16 and p21, have been identified recently and shown to be capable of binding to and inhibiting the activities of various CDK enzymes (for a recent review, see Ref. 5). The p16INK4 family of CDK inhibitors specifically interacts with two closely related CDK proteins, CDK4 and CDK6, both of which have been strongly implicated as the physiological pRb kinases. One member of this family, p15 INK4B, was specifica...

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PurposeTo enhance an entrepreneur's business network through the integration of the social network concepts and design principles of virtual communities.Design/methodology/approachThis study documents the design and initial deployment of a virtual community case, Innovation Information Infrastructure, based on social network concepts.FindingsBasic design principles, deployment strategy, and future directions for social network‐based virtual communities are presented.Research limitations/implicationsBecause of resource and time constraints, only basic content service, communication tools, and transaction functions were implemented in the initial deployment. Future extensions of this study may include development of a personalized and intelligent information retrieval system utilizing data mining techniques, development of advanced communication features to promote active participation, and creation of automatic social network‐tracking tools to monitor an individual's network evolution.Practical implicationsSample designs are provided for researchers who are interested in conducting a similar study.Originality/valueThis study serves as a starting‐point for how theories from social network and virtual community research streams can be applied in creating a virtual community that facilitates transformation of knowledge into products and services, which in turn lead to greater economic prosperity.