title: Tuberin binds p27 and negatively regulates its interaction with the SCF component Skp2.
abstract: TSC1 (tuberous sclerosis complex 1) encoding hamartin and TSC2 encoding tuberin are tumor suppressor genes responsible for the autosomal dominantly inherited disease tuberous sclerosis. These genes have been demonstrated to negatively regulate cell cycle progression, the activity of cdk2, and the degradation of the cyclin-dependent kinase inhibitor p27. To date, the underlying molecular mechanism remains elusive. Here, we show that tuberin binds to p27. Whereas tuberin also binds p27 in TSC1-negative cells, hamartin does not bind p27 without tuberin. p27 protein levels are regulated through ubiquitin-dependent degradation. Skp2 is the F-box protein, which, together with other proteins, forms an SCF (Skp1/cullin/F-box protein)-type E3 ubiquitin ligase complex whose task is to target p27 for degradation by the proteasome. We found that neither tuberin nor hamartin are in a complex with Skp2. Tuberin does not affect Skp2 protein levels, and the SCFSkp2 ubiquitin ligase does not regulate tuberin stability. But binding of tuberin to p27 sequesters p27 from Skp2 accompanied by an up-regulation of the p27 interaction with cdk2. Skp2-induced p27 degradation and cell cycle progression is abolished by tuberin's protective binding to p27. This work, the first description of the direct interaction of a tumor suppressor protein with p27, provides a molecular explanation for the effects of tuberous sclerosis complex genes on the cell cycle and demonstrates a new aspect of the SCFSkp2-mediated regulation of p27 stability.
authors.affiliation: Medical University of Vienna, Obstetrics and Gynecology, Prenatal Diagnosis and Therapy, Währinger Gürtel 18-20, 1090 Vienna, Austria.
authors.forename: Margit
authors.initials: M
authors.lastname: Rosner
authors.forename: Markus
authors.initials: M
authors.lastname: Hengstschläger
mesh_terms: D000818:Animals; D002453:Cell Cycle; D018797:Cell Cycle Proteins / Q000737:chemistry* / Q000378:metabolism; D002460:Cell Line; D045744:Cell Line, Tumor; D050760:Cyclin-Dependent Kinase Inhibitor p27; D018076:DNA, Complementary / Q000378:metabolism; D005347:Fibroblasts / Q000378:metabolism; D006367:HeLa Cells; D006801:Humans; D015151:Immunoblotting; D047468:Immunoprecipitation; D051379:Mice; D008954:Models, Biological; D046988:Proteasome Endopeptidase Complex / Q000378:metabolism; D011485:Protein Binding; D017434:Protein Structure, Tertiary; D011506:Proteins / Q000378:metabolism; D051381:Rats; D012097:Repressor Proteins / Q000737:chemistry / Q000502:physiology*; D044786:S-Phase Kinase-Associated Proteins / Q000378:metabolism*; D014162:Transfection; D000077004:Tuberous Sclerosis Complex 1 Protein; D000077005:Tuberous Sclerosis Complex 2 Protein; D025521:Tumor Suppressor Proteins / Q000737:chemistry* / Q000378:metabolism; D015854:Up-Regulation
chemical_list: C493961:Cdkn1b protein, mouse; C493962:Cdkn1b protein, rat; D018797:Cell Cycle Proteins; D018076:DNA, Complementary; D011506:Proteins; D012097:Repressor Proteins; D044786:S-Phase Kinase-Associated Proteins; C000624650:TSC1 protein, human; C000624653:TSC2 protein, human; C000624651:Tsc1 protein, mouse; C000624652:Tsc1 protein, rat; C000624654:Tsc2 protein, mouse; C000624655:Tsc2 protein, rat; D000077004:Tuberous Sclerosis Complex 1 Protein; D000077005:Tuberous Sclerosis Complex 2 Protein; D025521:Tumor Suppressor Proteins; D050760:Cyclin-Dependent Kinase Inhibitor p27; D046988:Proteasome Endopeptidase Complex