Abstract: Ubiquitin-mediated destruction of regulatory proteins is a frequent means of controlling progression through signaling pathways [[1]Hershko A Ciechanover A The ubiquitin system.Annu Rev Biochem. 1998; 67: 425-479Crossref PubMed Scopus (6471) Google Scholar]. F-box proteins [[2]Bai C Sen P Mathias N Hofmann K Ma L Goebl M Harper JW Elledge SJ SKP1 connects cell cycle regulators to the ubiquitin proteolysis machinery through a novel motif, the F-box.Cell. 1996; 86: 263-274Abstract Full Text Full Text PDF PubMed Scopus (932) Google Scholar] are components of modular E3 ubiquitin protein ligases called SCFs, which function in phosphorylation-dependent ubiquitination ([3Skowyra D Craig K Tyers M Elledge SJ Harper JW F-box proteins are components of E3 complexes and act as receptors to recruit phosphorylated substrates for ubiquitination.Cell. 1997; 91: 209-219Abstract Full Text Full Text PDF PubMed Scopus (1001) Google Scholar, 4Feldman RM Correll CC Kaplan KB Deshaies RJ A complex of Cdc4p, Skp1p, and Cdc53p/cullin catalyzes ubiquitination of the phosphorylated CDK inhibitor Sic1p.Cell. 1997; 91: 221-230Abstract Full Text Full Text PDF PubMed Scopus (701) Google Scholar, 5Skowyra D Koepp DM Kamura T Conrad MN Conaway RC Conaway JW Elledge SJ Harper JW Reconstitution of G1 cyclin ubiquitination with complexes containing SCFGrr1 and Rbx1.Science. 1999; 284: 662-665Crossref PubMed Scopus (350) Google Scholar], reviewed in [6Elledge SJ Harper JW Proteolysis in cell cycle control and cancer.Biochem Biophys Acta. 1998; 1377: M61-M70PubMed Google Scholar, 7Patton EE Willems AR Tyers M Combinatorial control in ubiquitin-dependent proteolysis: don't Skp the F-box hypothesis.Trends Genet. 1998; 14: 236-243Abstract Full Text Full Text PDF PubMed Scopus (439) Google Scholar]). F-box proteins contain a carboxy-terminal domain that interacts with substrates and a 42–48 amino-acid F-box motif which binds to the protein Skp1 [2Bai C Sen P Mathias N Hofmann K Ma L Goebl M Harper JW Elledge SJ SKP1 connects cell cycle regulators to the ubiquitin proteolysis machinery through a novel motif, the F-box.Cell. 1996; 86: 263-274Abstract Full Text Full Text PDF PubMed Scopus (932) Google Scholar, 3Skowyra D Craig K Tyers M Elledge SJ Harper JW F-box proteins are components of E3 complexes and act as receptors to recruit phosphorylated substrates for ubiquitination.Cell. 1997; 91: 209-219Abstract Full Text Full Text PDF PubMed Scopus (1001) Google Scholar, 4Feldman RM Correll CC Kaplan KB Deshaies RJ A complex of Cdc4p, Skp1p, and Cdc53p/cullin catalyzes ubiquitination of the phosphorylated CDK inhibitor Sic1p.Cell. 1997; 91: 221-230Abstract Full Text Full Text PDF PubMed Scopus (701) Google Scholar]. Skp1 binding links the F-box protein with a core ubiquitin ligase composed of the proteins Cdc53/Cul1, Rbx1 (also called Hrt1 and Roc1) and the E2 ubiquitin-conjugating enzyme Cdc34 [8Patton EE Willems AR Sa D Kuras L Thomas D Craig KL Tyers M Cdc53 is a scaffold protein for multiple Cdc34/Skp1/F-box protein complexes that regulate cell division and methionine biosynthesis in yeast.Genes Dev. 1998; 12: 692-705Crossref PubMed Scopus (232) Google Scholar, 9Kamura T Koepp DM Conrad MN Skowyra D Moreland RJ Iliopoulos O et al.Rbx1, a component of the VHL tumor suppressor complex and SCF ubiquitin ligase.Science. 1999; 284: 657-661Crossref PubMed Scopus (643) Google Scholar, 10Seol JH Feldman RM Zachariae W Shevchenko A Correll CC Lyapina S et al.Cdc53/cullin and the essential hrt1 RING-H2 subunit of SCF define a ubiquitin ligase module that activates the E2 enzyme cdc34.Genes Dev. 1999; 13: 1614-1626Crossref PubMed Scopus (348) Google Scholar, 11Ohta T Michel JJ Schottelius AJ Xiong Y ROC1, a homolog of APC11, represents a family of cullin partners with an associated ubiquitin ligase activity.Mol Cell. 1999; 3: 535-541Abstract Full Text Full Text PDF PubMed Scopus (393) Google Scholar]. The genomes of the budding yeast Saccharomyces cerevisiae and the nematode worm Caenorhabditis elegans contain, respectively, 16 and more than 60 F-box proteins [2Bai C Sen P Mathias N Hofmann K Ma L Goebl M Harper JW Elledge SJ SKP1 connects cell cycle regulators to the ubiquitin proteolysis machinery through a novel motif, the F-box.Cell. 1996; 86: 263-274Abstract Full Text Full Text PDF PubMed Scopus (932) Google Scholar, 7Patton EE Willems AR Tyers M Combinatorial control in ubiquitin-dependent proteolysis: don't Skp the F-box hypothesis.Trends Genet. 1998; 14: 236-243Abstract Full Text Full Text PDF PubMed Scopus (439) Google Scholar]; in S. cerevisiae, the F-box proteins Cdc4, Grr1 and Met30 target cyclin-dependent kinase inhibitors, G1 cyclins and transcriptional regulators for ubiquitination ([3Skowyra D Craig K Tyers M Elledge SJ Harper JW F-box proteins are components of E3 complexes and act as receptors to recruit phosphorylated substrates for ubiquitination.Cell. 1997; 91: 209-219Abstract Full Text Full Text PDF PubMed Scopus (1001) Google Scholar, 4Feldman RM Correll CC Kaplan KB Deshaies RJ A complex of Cdc4p, Skp1p, and Cdc53p/cullin catalyzes ubiquitination of the phosphorylated CDK inhibitor Sic1p.Cell. 1997; 91: 221-230Abstract Full Text Full Text PDF PubMed Scopus (701) Google Scholar, 5Skowyra D Koepp DM Kamura T Conrad MN Conaway RC Conaway JW Elledge SJ Harper JW Reconstitution of G1 cyclin ubiquitination with complexes containing SCFGrr1 and Rbx1.Science. 1999; 284: 662-665Crossref PubMed Scopus (350) Google Scholar, 8Patton EE Willems AR Sa D Kuras L Thomas D Craig KL Tyers M Cdc53 is a scaffold protein for multiple Cdc34/Skp1/F-box protein complexes that regulate cell division and methionine biosynthesis in yeast.Genes Dev. 1998; 12: 692-705Crossref PubMed Scopus (232) Google Scholar, 10Seol JH Feldman RM Zachariae W Shevchenko A Correll CC Lyapina S et al.Cdc53/cullin and the essential hrt1 RING-H2 subunit of SCF define a ubiquitin ligase module that activates the E2 enzyme cdc34.Genes Dev. 1999; 13: 1614-1626Crossref PubMed Scopus (348) Google Scholar], reviewed in [6Elledge SJ Harper JW Proteolysis in cell cycle control and cancer.Biochem Biophys Acta. 1998; 1377: M61-M70PubMed Google Scholar, 7Patton EE Willems AR Tyers M Combinatorial control in ubiquitin-dependent proteolysis: don't Skp the F-box hypothesis.Trends Genet. 1998; 14: 236-243Abstract Full Text Full Text PDF PubMed Scopus (439) Google Scholar]). Only four mammalian F-box proteins (Cyclin F, Skp1, β-TRCP and NFB42) have been identified so far [2Bai C Sen P Mathias N Hofmann K Ma L Goebl M Harper JW Elledge SJ SKP1 connects cell cycle regulators to the ubiquitin proteolysis machinery through a novel motif, the F-box.Cell. 1996; 86: 263-274Abstract Full Text Full Text PDF PubMed Scopus (932) Google Scholar, 12Erhardt JA Hynicka W DiBenedetto A Shen N Stone N Paulson H Pittman RN A novel F box protein, NFB42, is highly enriched in neurons and induces growth arrest.J Biol Chem. 1998; 273: 35222-35227Crossref PubMed Scopus (52) Google Scholar]. Here, we report the identification of a family of 33 novel mammalian F-box proteins. The large number of these proteins in mammals suggests that the SCF system controls a correspondingly large number of regulatory pathways in vertebrates. Four of these proteins contain a novel conserved motif, the F-box-associated (FBA) domain, which may represent a new protein–protein interaction motif. The identification of these genes will help uncover pathways controlled by ubiquitin-mediated proteolysis in mammals.