A family of mammalian F-box proteins

Abstract

Ubiquitin-mediated destruction of regulatory proteins is a frequent means of controlling progression through signaling pathways [1]. F-box proteins [2] are components of modular E3 ubiquitin protein ligases called SCFs, which function in phosphorylation-dependent ubiquitination ([3] [4] [5], reviewed in [6] [7]). 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 [2] [3] [4]. 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 [8] [9] [10] [11]. The genomes of the budding yeast Saccharomyces cerevisiae and the nematode worm Caenorhabditis elegans contain, respectively, 16 and more than 60 F-box proteins [2] [7]; in S. cerevisiae, the F-box proteins Cdc4, Grr1 and Met30 target cyclin-dependent kinase inhibitors, G1 cyclins and transcriptional regulators for ubiquitination ([3] [4] [5] [8] [10], reviewed in [6] [7]). Only four mammalian F-box proteins (Cyclin F, Skp1, beta-TRCP and NFB42) have been identified so far [2] [12]. 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.

Involvement of CUL4 Ubiquitin E3 Ligases in Regulating CDK Inhibitors Dacapo/p27Kip1 and Cyclin E Degradation

“…Although in human cells, SCF SKP2 regulates p27, there is no structural and functional evidence that SKP2 is conserved in Drosophila cells. 2,32,33 In addition, although Regulation The SD2 cells were transfected with dsRNAs for Neo, CUL1, CUL4, Dacapo, CUL4+Dacapo, CSN1, and CSN1+Dacapo for 60 hours as in Figure 1A. The levels of Drosophila CUL4, CUL1, Armadillo, cyclin E, Dacapo, and CAND1 were analyzed by western blotting using their respective antibodies as indicated.…”

Inhibition of HOS expression and activities by Wnt pathway

“…Whereas speci®c phosphorylation of a DS(PO 3 )GcXS(PO 3 ) motif in IkB (by IkB kinase) and in b-catenin (by GSK3b) is indispensable, the activity of the ligase is also essential for ubiquitination of those substrates. SCF HOS/bTrCP E3 ubiquitin ligase consists of one of the two closely related F-box proteins (bTrCP/Fbw1a and HOS/bTrCP2/Fbw1b) (Cenciarelli et al, 1999;Winston et al, 1999a) as well as of Skp1, Cull and ROC1/Rbx1/Hrt1 proteins (reviewed in Deshaies, 1999).…”

PP2ACdc55regulates G1cyclin stability

“…GRR1 is not essential, while CDC4 and MET30 are required for viability. 36,37 To further define F-box specificity, a genetic sick phenotype was tested for between loss of CDC55 and recessive alleles of CDC4 or MET30. We asked if the loss of CDC55 reduced the temperature required for the cell death of cdc4-1 ts or met30-6 ts cells.…”