title: Dimers initiate and propagate serine protease inhibitor polymerisation.
abstract: The serine protease inhibitor (serpin) family can readily form long-chain polymers by a process that underlies a variety of diseases. We show here that monomers of plasma serpins alpha(1)-antitrypsin and antithrombin are stable on incubation with the rate-limiting step in their polymerisation being the formation of the initial dimer. Once formed, the dimers readily interlink to form tetramers and can bind monomers to form trimers and longer oligomers. Cleavage of the only exposed reactive loop, in unit I of the dimers, prevents their interlinkage, but these cleaved dimers can still link to monomers. The rapid binding by the cleaved dimers of a peptide specific to the lower half of beta-sheet A of the molecule indicates the ready opening of this beta-sheet in unit II of the dimers. The failure of the cleaved dimers to bind peptide-complexed monomers, together with the relative inaccessibility of the P14 hinge residue in the oligomers, is evidence that partial insertion of the reactive loop into its own A-sheet is required for polymer formation. We propose that serpin dimers initiate and propagate polymerisation by having one exposed loop with an optimal conformation as a beta-strand donor and a readily opened beta-sheet as an acceptor. The sequential reformation of these activated beta-interfaces as the oligomer extends, molecule by molecule, provides a model for the fibril and amyloid formation of conformational diseases in general as well as for the infectivity of prion encephalopathies.
authors.affiliation: Department of Haematology, Cambridge Institute for Medical Research, University of Cambridge, Hills Road, Cambridge CB2 2XY, UK. awz20@cam.ac.uk
authors.forename: Aiwu
authors.initials: A
authors.lastname: Zhou
authors.forename: Robin W
authors.initials: RW
authors.lastname: Carrell
mesh_terms: D000991:Antithrombins / Q000737:chemistry / Q000235:genetics / Q000302:isolation & purification / Q000378:metabolism; D001710:Biotin / Q000378:metabolism; D019281:Dimerization; D004591:Electrophoresis, Polyacrylamide Gel; D006358:Hot Temperature; D006801:Humans; D006863:Hydrogen-Ion Concentration; D008954:Models, Biological; D008958:Models, Molecular; D010446:Peptide Fragments / Q000737:chemistry; D011485:Protein Binding; D011487:Protein Conformation; D017433:Protein Structure, Secondary; D011994:Recombinant Proteins / Q000097:blood / Q000737:chemistry / Q000302:isolation & purification / Q000378:metabolism; D015842:Serine Proteinase Inhibitors / Q000737:chemistry* / Q000378:metabolism*; D013997:Time Factors; D000515:alpha 1-Antitrypsin / Q000097:blood / Q000737:chemistry / Q000235:genetics / Q000302:isolation & purification / Q000378:metabolism
chemical_list: D000991:Antithrombins; D010446:Peptide Fragments; D011994:Recombinant Proteins; D015842:Serine Proteinase Inhibitors; D000515:alpha 1-Antitrypsin; D001710:Biotin