The abundance of cell cycle regulatory protein Cdc4p is controlled by interactions between its F box and Skp1p

Posttranslational modification of a protein by ubiquitin usually results in rapid degradation of the ubiquitinated protein by the proteasome. The tran sfer of ubiquitin to substrate is a multistep process. Cdc4p is a component of a ubiquitin ligase that tethers the ubiquitin-conjugating enzyme Cdc34p to its substrates. Among the domains of Cdc4p that are crucial for functio n are the F-box, which links Cdc4p to Cdc53p through Skp1p, and the WD-40 r epeats, which are required for binding the substrate for Cdc34p. In additio n to Cdc4p, other F-box proteins, including Grr1p and Met30p, may similarly act together with Cdc53p and Skp1p to function as ubiquitin ligase complex es. Because the relative abundance of these complexes, known collectively a s SCFs, is important for cell viability, we have sought evidence of mechani sms that modulate F-box protein regulation. Here we demonstrate that the ab undance of Cdc4p is subject to control by a peptide segment that me term th e R-motif (for "reduced abundance"). Furthermore, we show that binding of S kp1p to the F-box of Cdc4p inhibits R-motif-dependent degradation of Cdc4p. These results suggest a general model for control of SCF activities.