High-resolution epitope mapping by HX MS reveals the pathogenic mechanism and a possible therapy for autoimmune TTP syndrome.

Acquired thrombotic thrombocytopenic purpura (TTP), a thrombotic disorder that is fatal in almost all cases if not treated promptly, is primarily caused by IgG-type autoantibodies that inhibit the ability of the ADAMTS13 (a disintegrin and metalloproteinase with a thrombospondin type 1 motif, member 13) metalloprotease to cleave von Willebrand factor (VWF). Because the mechanism of autoantibody-mediated inhibition of ADAMTS13 activity is not known, the only effective therapy so far is repeated whole-body plasma exchange. We used hydrogen-deuterium exchange mass spectrometry (HX MS) to determine the ADAMTS13 binding epitope for three representative human monoclonal autoantibodies, isolated from TTP patients by phage display as tethered single-chain fragments of the variable regions (scFvs). All three scFvs bind the same conformationally discontinuous epitopic region on five small solvent-exposed loops in the spacer domain of ADAMTS13. The same epitopic region is also bound by most polyclonal IgG autoantibodies in 23 TTP patients that we tested. The ability of ADAMTS13 to proteolyze VWF is impaired by the binding of autoantibodies at the epitopic loops in the spacer domain, by the deletion of individual epitopic loops, and by some local mutations. Structural considerations and HX MS results rule out any disruptive structure change effect in the distant ADAMTS13 metalloprotease domain. Instead, it appears that the same ADAMTS13 loop segments that bind the autoantibodies are also responsible for correct binding to the VWF substrate. If so, the autoantibodies must prevent VWF proteolysis simply by physically blocking normal ADAMTS13 to VWF interaction. These results point to the mechanism for autoantibody action and an avenue for therapeutic intervention.

Light chain of factor VIII is sufficient for accelerating cleavage of von Willebrand factor by ADAMTS13 metalloprotease.

We previously demonstrated that coagulation factor VIII (FVIII) accelerates proteolytic cleavage of von Willebrand factor (VWF) by A disintegrin and metalloprotease with thrombospondin type 1 repeats (ADAMTS13) under fluid shear stress. In this study, the structural elements of FVIII required for the rate-enhancing effect and the biological relevance of this cofactor activity are determined using a murine model. An isolated light chain of human FVIII (hFVIII-LC) increases proteolytic cleavage of VWF by ADAMTS13 under shear in a concentration-dependent manner. The maximal rate-enhancing effect of hFVIII-LC is ∼8-fold, which is comparable with human full-length FVIII and B-domain deleted FVIII (hFVIII-BDD). The heavy chain (hFVIII-HC) and the light chain lacking the acidic (a3) region (hFVIII-LCΔa3) have no effect in accelerating VWF proteolysis by ADAMTS13 under the same conditions. Although recombinant hFVIII-HC and hFVIII-LCΔa3 do not detectably bind immobilized VWF, recombinant hFVIII-LC binds VWF with high affinity (K(D), ∼15 nM). Moreover, ultra-large VWF multimers accumulate in the plasma of fVIII(-/-) mice after hydrodynamic challenge but not in those reconstituted with either hFVIII-BDD or hFVIII-LC. These results suggest that the light chain of FVIII, which is not biologically active for clot formation, is sufficient for accelerating proteolytic cleavage of VWF by ADAMTS13 under fluid shear stress and (patho) physiological conditions. Our findings provide novel insight into the molecular mechanism of how FVIII regulates VWF homeostasis.

Role for a conserved structural motif in assembly of a class I aminoacyl-tRNA synthetase active site.

The catalytic domains of class I aminoacyl-tRNA synthetases are built around a conserved Rossmann nucleotide binding fold, with additional polypeptide domains responsible for tRNA binding or hydrolytic editing of misacylated substrates. Structural comparisons identified a conserved motif bridging the catalytic and anticodon binding domains of class Ia and Ib enzymes. This stem contact fold (SCF) has been proposed to globally orient each enzyme's cognate tRNA by interacting with the inner corner of the L-shaped tRNA. Despite the structural similarity of the SCF among class Ia/Ib enzymes, the sequence conservation is low. We replaced amino acids of the MetRS SCF with portions of the structurally similar glutaminyl-tRNA synthetase (GlnRS) motif or with alanine residues. Chimeric variants retained significant tRNA methionylation activity, indicating that structural integrity of the helix-turn-strand-helix motif contributes more to tRNA aminoacylation than does amino acid identity. In contrast, chimeras were significantly reduced in methionyl adenylate synthesis, suggesting a role for the SCF in formation of a structured active site domain. A highly conserved aspartic acid within the MetRS SCF is proposed to make an electrostatic interaction with an active site lysine; these residues were replaced with alanines or conservative substitutions. Both methionyl adenylate formation and methionine transfer were impaired, and activity was not significantly recovered by making the compensatory double substitution.