Modification of Cul1 regulates its association with proteasomal subunits

Ubiquitylation targets proteins for degradation by the 26S proteasome. Some yeast and plant ubiquitin ligases, including the highly conserved SCF (S kp1/C ul1/F-box protein) complex, have been shown to associate with proteasomes. We sought to characterize interactions between SCF complexes and proteasomes in mammalian cells. We found that the binding of SCF complexes to proteasomes is conserved in higher eukaryotes. The Cul1 subunit associated with both sub-complexes of the proteasome, and high molecular weight forms of Cul1 bound to the 19S proteasome. Cul1 is ubiquitylated in vivo. Ubiquitylation of Cul1 promotes its binding to the S5a subunit of the 19S sub-complex without affecting Cul1 stability. The association of ubiquitylating enzymes with proteasomes may be an additional means to target ubiquitylated substrates for degradation.