We have reconstituted the ubiquitination pathway for the Cdk inhibitor Sic1 using recombinant proteins. Skp1, Cdc53, and the F-box protein Cdc4 form a complex, SCFCdc4, which functions as a Sic1 ubiquitin-ligase (E3) in combination with the ubiquitin conjugating enzyme (E2) Cdc34 and E1. Cdc4 assembled with Skp1 functions as the receptor that selectively binds phosphorylated Sic1. Grr1, an F-box protein involved in Cln destruction, forms complexes with Skp1 and Cdc53 and binds phosphorylated Cln1 and Cln2, but not Sic1. Because the constituents of the SCF complex are members of protein families, SCFCdc4 is likely to serve as the prototype for a large class of E3s formed by combinatorial interactions of related family members. SCF complexes couple protein kinase signaling pathways to the control of protein abundance.