CDC34 encodes a ubiquitin-conjugating enzyme (Ubc/E2) that catalyzes Skp1-Cullin-F-box (SCF) ubiquitin protein ligase-mediated substrate ubiquitination (3, 6, 7). SCF-mediated ubiquitination involves three sequential enzymatic steps: ATP-dependent activation of ubiquitin through the formation of a high-energy thioester linkage with a ubiquitin-activating enzyme (E1); transfer of the activated ubiquitin to a ubiquitin-conjugating enzyme (E2); and the E2-catalyzed transfer of activated ubiquitin to a specific lysine residue(s) of the target protein, aided by substrate-specific components of SCF ubiquitin protein ligase complexes (E3s). This sequence is repeated until multiple chains of ubiquitin are attached, thereby marking the protein for rapid degradation by the 26S proteasome (reviewed in 8 and 4).

SCF ubiquitin protein ligases are RING-H2 type ligase family members that function as target recognition modules, bringing Cdc34p and substrates into close proximity, thereby positioning Cdc34p for efficient transfer of activated ubiquitin (reviewed in 4, 9 and 10). SCF ubiquitin ligase core complexes are composed of several shared subunits including Skp1p, an adaptor protein that binds and recruits a variety of F-box containing proteins (11, 7); Cdc53p, a cullin family member that recruits Cdc34p to Skp1p/F-box proteins (12, 13, 14); Hrt1p, a RING-H2 domain protein that stimulates ubiquitin-ligase activity (15, 16); and Cdc34p, a ubiquitin-conjugating enzyme that catalyzes the transfer of activated ubiquitin to the target protein (3, 6, 7). In addition to these shared subunits SCF complexes also contain one of several unique F-box motif containing proteins (Cdc4p, Grr1p, Met30p, Dia2p, or Saf1p) that function as substrate-specific adaptors or specificity determinants recruiting multiply phosphorylated substrates to the SCF core complex (reviewed in 4, 17 and 18). Multiple SCF-F-box specific substrates have been identified. SCF-Cdc4p facilitates the polyubiquitination of Sic1p (6, 7), Far1p (19, 20), Cdc6p (21, 22), Clb6p (23) and Hac1p (24); substrates of SCF-Grr1p include Cln1p, Cln2p (13, 25, 26), Gic1p, and Gic2p (27); substrates of SCF-Met30p include Met4p (28) and Swe1p (29); SCF-Dia2p polyubiquitinates Tec1p (30); and SCF-Saf1p polyubiquitinates Aah1p (31, 32). The F-box proteins Cdc4p, Grr1p, and Met30p are also intrinsically unstable and are able to catalyze their own SCF-mediated ubiquitination and destruction via an autocatalytic mechanism proposed to facilitate rapid switching among multiple SCF complexes (33, 34). As a component of the SCF core complex, Cdc34p has been directly implicated in the ubiquitin-mediated proteolysis of all of the substrates listed above (26, 13, 12, 35, 21, 23, 12, 36, 29, 37, 12, 19, 24, 27, 30, 31).

cdc34 was originally identified as a cell division cycle (CDC) mutant that arrests at the G1/S phase transition with multiple elongated buds and unreplicated DNA, similar to cdc4, cdc53 and hrt1 mutants (14, 3, 16, 38). Deletion of the CDK inhibitor SIC1 alters the terminal phenotype of cdc34, cdc53 and cdc4 mutants, resulting in cells that now arrest at the G2/M phase transition of the cell cycle with replicated DNA and a single round bud (27, 39). It is therefore the accumulation of this SCF substrate (Sic1p) that is solely responsible for the failure of these mutants to enter into S phase, and emphasizes the importance of ubiquitin-mediated proteolysis for regulated passage through key cell cycle transitions.

Cdc34p is regulated by phosphorylation, autoubiquitination and self-association (38, 40, 41). The carboxy-terminal tail of Cdc34p is phosphorylated on serine residues by casein kinase 2 (a tetrameric enzyme containing Cka1p, Cka2p, Ckb1p, and Ckb2p), and phophorylation regulates the intrinsic catalytic activity of the enzyme and its activity when assayed in the presence of SCF-Cdc4p (42). The modulation of Cdc34p activity in turn affects the expression of methionine (MET) biosynthetic genes (43, 44) as well as cell cycle progression (42), due to effects on Cdc34p substrates. Cdc34p is also able to catalyze its own ubiquitination via a mechanism that involves intramolecular transfer of the thiol-ester linked ubiquitin to a C-terminal lysine residue (40). Autoubiquitination of Cdc34p is stimulated by SCF, or by a Cdc53p-Hrt1p subcomplex (26, 15), although the regulatory importance of this modification has yet to be determined. Cdc34p interacts with itself in a ubiquitin-thiolester formation dependent manner and self-association is required for both its catalytic activity and its cell cycle function (41, 45).

CDC34 is evolutionarily conserved and a human CDC34 homolog (OMIM) has been identified based on functional complementation (46, 47). The subunit composition of the human SCF ubiquitin protein ligase is also conserved (48), and like Cdc34p, the human protein is phosphorylated by casein kinase 2 (49). Substrates of the human protein include oncoproteins and tumor suppressors (47, 50, 51).

Last updated: 2007-06-14