Three different binding sites of Cks1 are required for p27-ubiquitin ligation.
Previous studies have shown that the cyclin-dependent kinase (Cdk) inhibitor p27(Kip1) is targeted for degradation by an SCF(Skp2) ubiquitin ligase complex and that this process requires Cks1, a member of the highly conserved Suc1/Cks family of cell cycle regulatory proteins. All proteins of this family have Cdk-binding and anion-binding sites, but only mammalian Cks1 binds to Skp2 and promotes the association of Skp2 with p27 phosphorylated on Thr-187. The molecular mechanisms by which Cks1 promotes the interaction of the Skp2 ubiquitin ligase subunit to p27 remained obscure. Here we show that the Skp2-binding site of Cks1 is located on a region including the alpha2- and alpha1-helices and their immediate vicinity, well separated from the other two binding sites. All three binding sites of Cks1 are required for p27-ubiquitin ligation and for the association of Skp2 with Cdk-bound, Thr-187-phosphorylated p27. Cks1 and Skp2 mutually promote the binding of each other to a peptide similar to the 19 C-terminal amino acids of p27 containing phosphorylated Thr-187. This latter process requires the Skp2- and anion-binding sites of Cks1, but not its Cdk-binding site. It is proposed that the Skp2-Cks1 complex binds initially to the C-terminal region of phosphorylated p27 in a process promoted by the anion-binding site of Cks1. The interaction of Skp2 with the substrate is further strengthened by the association of the Cdk-binding site of Cks1 with Cdk2/cyclin E, to which phosphorylated p27 is bound.
Unit of Biochemistry, the B. Rappaport Faculty of Medicine, Technion-Israel Institute of Technology, Haifa 31096, Israel.
Danielle
D
Sitry
Markus A
MA
Seeliger
Tun K
TK
Ko
Dvora
D
Ganoth
Sadie E
SE
Breward
Laura S
LS
Itzhaki
Michele
M
Pagano
Avram
A
Hershko
D048868:Adaptor Proteins, Signal Transducing; D000595:Amino Acid Sequence; D001665:Binding Sites; D042846:CDC2-CDC28 Kinases; D018797:Cell Cycle Proteins / Q000378:metabolism; D051357:Cyclin-Dependent Kinase 2; D018844:Cyclin-Dependent Kinases / Q000378:metabolism; D005656:Fungal Proteins / Q000737:chemistry* / Q000378:metabolism; D006801:Humans; D008840:Microfilament Proteins / Q000378:metabolism*; D008969:Molecular Sequence Data; D009124:Muscle Proteins; D010766:Phosphorylation; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism; D044786:S-Phase Kinase-Associated Proteins; D029701:Saccharomyces cerevisiae Proteins; D025801:Ubiquitin / Q000378:metabolism*
D048868:Adaptor Proteins, Signal Transducing; C081185:CKS1 protein, S cerevisiae; D018797:Cell Cycle Proteins; D005656:Fungal Proteins; D008840:Microfilament Proteins; D009124:Muscle Proteins; D044786:S-Phase Kinase-Associated Proteins; D029701:Saccharomyces cerevisiae Proteins; C058992:Tagln protein, mouse; D025801:Ubiquitin; D017346:Protein Serine-Threonine Kinases; D042846:CDC2-CDC28 Kinases; C495894:CDK2 protein, human; D051357:Cyclin-Dependent Kinase 2; D018844:Cyclin-Dependent Kinases