Combinatorial diversity of fission yeast SCF ubiquitin ligases by homo- and heterooligomeric assemblies of the F-box proteins Pop1p and Pop2p

BACKGROUND: SCF ubiquitin ligases share the core subunits cullin 1, SKP1, and HRT1/RBX1/ROC1, which associate with different F-box proteins. F-box proteins bind substrates following their phosphorylation upon stimulation of various signaling pathways. Ubiquitin-mediated destruction of the fission yeast cyclin-dependent kinase inhibitor Rum1p depends on two heterooligomerizing F-box proteins, Pop1p and Pop2p. Both proteins interact with the cullin Pcu1p when overexpressed, but it is unknown whether this reflects their co-assembly into bona fide SCF complexes. RESULTS: We have identified Psh1p and Pip1p, the fission yeast homologues of human SKP1 and HRT1/RBX1/ROC1, and show that both associate with Pop1p, Pop2p, and Pcu1p into a ~500 kDa SCF(Pop1p-Pop2p) complex, which supports polyubiquitylation of Rum1p. Only the F-box of Pop1p is required for SCF(Pop1p-Pop2p) function, while Pop2p seems to be attracted into the complex through binding to Pop1p. Since all SCF(Pop1p-Pop2p) subunits, except for Pop1p, which is exclusively nuclear, localize to both the nucleus and the cytoplasm, the F-box of Pop2p may be critical for the assembly of cytoplasmic SCF(Pop2p) complexes. In support of this notion, we demonstrate individual SCF(Pop1p) and SCF(Pop2p) complexes bearing ubiquitin ligase activity. CONCLUSION: Our data suggest that distinct homo- and heterooligomeric assemblies of Pop1p and Pop2p generate combinatorial diversity of SCF(Pop) function in fission yeast. Whereas a heterooligomeric SCF(Pop1p-Pop2p) complex mediates polyubiquitylation of Rum1p, homooligomeric SCF(Pop1p) and SCF(Pop2p) complexes may target unknown nuclear and cytoplasmic substrates.