Structural analysis of a function-associated loop mutant of the substrate-recognition domain of Fbs1 ubiquitin ligase

The SCF ubiquitin ligase comprises four components: Skp1, Cul1, Rbx1 and a variable-subunit F-box protein. The F-box protein Fbs1, which recognizes the N-linked glycoproteins, is involved in the endoplasmic reticulum-associated degradation pathway. Although FBG3, another F-box protein, shares 51% sequence identity with Fbs1, FBG3 does not bind glycoproteins. To investigate the sequence–structure relationship of the substrate-binding pocket, the crystal structure of a mutant substrate-binding domain of Fbs1 in which the six nonconserved regions (β1, β2–β3, β3–β4, β5–β6, β7–β8 and β9–β10) of Fbs1 were substituted with those of FBG3 was determined. The substrate-binding pocket of this model exhibits structural features that differ from those of Fsb1.