An antibody that prevents serpin polymerisation acts by inducing a novel allosteric behaviour

Serpins are important regulators of proteolytic pathways with an antiprotease activity thatinvolves a conformational transition from a metastable to a hyperstable state. Certainmutations permit the transition to occur in the absence of a protease; when associatedwith an intermolecular interaction, this yields linear polymers of hyperstable serpin mole-cules, which accumulate at the site of synthesis. This is the basis of many pathologiestermed the serpinopathies. We have previously identified a monoclonal antibody(mAb4B12) that, in single-chain form, blocksα1-antitrypsin (α1-AT) polymerisation in cells.Here, we describe the structural basis for this activity. The mAb4B12epitope was found toencompass residues Glu32, Glu39 and His43 on helix A and Leu306 on helix I. This isnot a region typically associated with the serpin mechanism of conformational change,and correspondingly the epitope was present in all tested structural forms of the protein.Antibody binding renderedβ-sheet A—on the opposite face of the molecule—moreliable to adopt an‘open’state, mediated by changes distal to the breach region andproximal to helix F. The allosteric propagation of induced changes through the moleculewas evidenced by an increased rate of peptide incorporation and destabilisation of a pre-formed serpin–enzyme complex following mAb4B12binding. These data suggest that pre-maturely shifting theβ-sheet A equilibrium towards the‘open’state out of sequence withother changes suppresses polymer formation. This work identifies a region potentiallyexploitable for a rational design of ligands that is able to dynamically influenceα1-ATpolymerisation.