The cell-cycle regulatory protein Cks1 is required for SCFSkp2-mediated ubiquitinylation of p27

The cyclin-dependent kinase (CDK) inhibitor p27 is degraded in late G1 phas e by the ubiquitin pathway(1), allowing CDK activity to drive cells into S phase(2). Ubiquitinylation of p27 requires its phosphorylation at Thr 187 ( refs 3, 4) and subsequent recognition by S-phase kinase associated protein 2 (Skp2; refs 5-8), a member of the F-box family of proteins that associate s with Skp1, Cul-1 and ROC1/Rbx1 to form an scF ubiquitin ligase complex(9) . However, in vitro ligation of p27 to ubiquitin could not be reconstituted by known purified components of the SCFSkp2 complex. Here we show that the missing factor is CDK subunit 1 (Cks1), which belongs to the highly conser ved Suc1/Cks family of proteins that bind to some CDKs and phosphorylated p roteins and are essential for cell-cycle progression. Human Cks1, but not o ther members of the family, reconstitutes ubiquitin ligation of p27 in a co mpletely purified system, binds to Skp2 and greatly increases binding of T1 87-phosphorylated p27 to Skp2. Our results represent the first evidence tha t an SCF complex requires an accessory protein for activity as well as for binding to its phosphorylated substrate.