Recognition of substrate and Skp1 by the Homologue of Slimb (HOS) ubiquitin ligase receptor D role of the F-box

SCFHOS-Roc1 E3 ubiquitin ligase is an enzymatic complex, which mediates ubiquitination and subsequent proteasome-dependent degradation of phosphorylated inhibitor of NF-kB (IkB) and b-catenin. HOS is a WD40 repeats/F-box-containing protein that actually associates with the substrates and binds to Skp1 via the F-box. Here, we have studied the structural determinants of the substrate recognition and ligase recruitment by HOS. The binding (pull-down and immunoprecipitation assays) and ubiquitination assays were performed in vitro with purified or partially purified recombinant proteins obtained via expression in bacteria or mammalian cells or by in vitro translation. We identified specific amino acid residues (I143 and L152) within the F-box of HOS that play a critical role in maintaining the hydrophobic interface of HOS-Skp1 interaction and found substantial similarity between interaction of Skp1 with HOS and with another F-box protein Skp2. Binding of Skp1 augments the ability of HOS to recognize the phosphorylated IkBa. These observations indicate the role of the F-box of HOS in both recruitment of ubiquitin ligase activity and substrate recognition as well as identify the structural elements that are important for both functions of HOS F-box domain.