Structural basis for the selection of glycosylated substrates by SCF ^Fbs1 ubiquitin ligase

Abstract

The ubiquitin ligase complex SCF(Fbs1), which contributes to the ubiquitination of glycoproteins, is involved in the endoplasmic reticulum-associated degradation pathway. In SCF ubiquitin ligases, a diverse array of F-box proteins confers substrate specificity. Fbs1/Fbx2, a member of the F-box protein family, recognizes high-mannose oligosaccharides. To elucidate the structural basis of SCF(Fbs1) function, we determined the crystal structures of the Skp1-Fbs1 complex and the sugar-binding domain (SBD) of the Fbs1-glycoprotein complex. The mechanistic model indicated by the structures appears to be well conserved among the SCF ubiquitin ligases. The structure of the SBD-glycoprotein complex indicates that the SBD primarily recognizes Man(3)GlcNAc(2), thereby explaining the broad activity of the enzyme against various glycoproteins. Comparison of two crystal structures of the Skp1-Fbs1 complex revealed the relative motion of a linker segment between the F-box and the SBD domains, which might underlie the ability of the complex to recognize different acceptor lysine residues for ubiquitination.

Diversity in Tissue Expression, Substrate Binding, and SCF Complex Formation for a Lectin Family of Ubiquitin Ligases

“…Glycans-Previous studies have shown that FBXO2 and FBXO6 bind glycoproteins containing N-linked high mannose glycans (3,23,26,29,33,34). This raises the following important question.…”

“…1A) (23,33). Because of the high degree of amino acid identity among the G domains of the FBA family members (supplemental Fig.…”

Mechanism of CRL4Cdt2, a PCNA-dependent E3 ubiquitin ligase

“…Fbx2 recognizes glycosylated proteins that are retrotranslocated from the ER into the cytosol and targets them for destruction (Yoshida et al 2002;Mizushima et al 2007). Finally, some ubiquitin ligases interact preferentially with substrates that are sumoylated (Perry et al 2008).…”

Structural Basis of Dimerization-dependent Ubiquitination by the SCFFbx4 Ubiquitin Ligase

“…F-box proteins are the specificity factors in SCF; they interact with Skp1 through the N-terminal ϳ40-residue F-box motif to recruit substrates through their variable C-terminal protein-protein interaction domains, including WD40 repeats (Fbxw subfamily), leucine-rich repeats (Fbxl subfamily), and other different or unknown domains (Fbxo subfamily) (15,16). Recently, a new subfamily within the Fbxo family that recognizes N-linked oligosaccharide, so-called Fbs, has been identified (10,17). The accumulated evidence suggests that the large number and rich diversity of F-box proteins not only allow the recruitment of numerous, diverse substrates but also position them optimally for the ubiquitination reaction, an important feature of the SCF E3s (13,14).…”