Clamping, Bending, and Twisting Inter-Domain Motions in the Misfold-Recognising Portion of UDP-Glucose: Glycoprotein Glucosyl-Transferase

UDP-glucose: glycoprotein glucosyltransferase (UGGT) is the glycoprotein folding checkpoint in the eukaryotic glycoprotein secretory pathway. The enzyme detects misfolded glycoproteins in the Endoplasmic Reticulum, and flags them for retention by re-glucosylating them on an N-linked glycan. The fit of a UGGT crystal structure to a negative stain electron microscopy reconstruction of UGGT in complex with an antibody suggests that the misfold-sensing N-terminal portion of UGGT and its C-terminal catalytic domain are tightly associated. Molecular Dynamics (MD) simulations capture UGGT in so far unobserved conformational states, and principal component analysis of the MD trajectories affords a description of UGGT's overall inter-domain motions, highlighting three types of inter-domain movements: bending, twisting and clamping. These inter-domain motions modify the accessible surface area of the enzyme's central saddle. We propose to name "Parodi limit" the maximum distance between a site of misfolding on a UGGT glycoprotein substrate and an N-linked glycan that monomeric UGGT can re-glucosylate on the same glycoprotein. MD simulations estimate the Parodi limit to be around 60-70 A. Re-glucosylation assays using UGGT deletion mutants suggest that the TRXL2 domain is necessary for activity against urea-misfolded bovine thyroglobulin. Our findings support a "one-size-fits-all adjustable spanner" substrate recognition model, with a crucial role for the TRXL2 domain in the recruitment of misfolded substrates.