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

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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.

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