F-BOX PROTEINS ARE RECEPTORS THAT RECRUIT PHOSPHORYLATED SUBSTRATES TO THE SCF UBIQUITIN-LIGASE COMPLEX

We have reconstituted the ubiquitination pathway for the Cdk inhibitor Sic1 using recombinant proteins. Skp1, Cdc53, and the F-box protein C dc4 form a complex, SCFCdc4, which functions as a Sic1 ubiquitin-ligas e (E3) in combination with the ubiquitin conjugating enzyme (E2) Cdc34 and E1. Cdc4 assembled with Skp1 functions as the receptor that selec tively binds phosphorylated Sic1. Grr1, an F-box protein involved in C ln destruction, forms complexes with Skp1 and Cdc53 and binds phosphor ylated Cln1 and Cln2, but not Sic1. Because the constituents of the SC F complex are members of protein families, SCFCdc4 in likely to serve as the prototype for a large class of E3s formed by combinatorial inte ractions of related family members. SCF complexes couple protein kinas e signaling pathways to the control of protein abundance.