Journal of Molecular Biology
How Small Peptides Block and Reverse Serpin Polymerisation
Introduction
The conformational diseases,1 which include the common dementias of Alzheimer's,2 Parkinson's3 and the spongiform encephalopathies,4 all result from aberrant β-interlinkages of an underlying protein. The serpin family of serine protease inhibitors provides the best-defined structural model of such β-linkages and of the consequent protein aggregation and disease.5, 6, 7 Mutations that affect the conformational stability of serpins such as the plasma protease inhibitors antithrombin and α1-antitrypsin (hereafter referred to as antitrypsin), lead to their polymerisation and intracellular aggregation. The accompanying loss of inhibitory activity results with antithrombin in a distinctive thrombotic disease,8 and with antitrypsin in the slow onset of the destructive lung disease, emphysema.9 But even more severe consequences arise from the concomitant cell damage due to the massive intracellular aggregation of the conformationally unstable serpins. This occurs with antitrypsin, where its accumulation within the hepatocyte leads to liver cirrhosis10 and with neuroserpin, where its intracellular accumulation is associated with a cumulative loss of neurons and the onset of an Alzheimer's-like dementia.11
The polymerisation of serpins results from an abnormality of their inhibitory mechanism. The inhibition of proteases by serpins requires the insertion of the first 14 residues of the cleaved reactive loop (P14-1) into the vacant strand 4 position of the main (A) β-sheet of the molecule.12, 13 Pathological molecular interlinkage occurs when the A-sheet aberrantly opens and allows a domain exchange with the insertion of the P8-3 portion of the loop of another molecule into the lower half of the s4A position, with sequential intermolecular linkages resulting in polymer or fibril formation14, 15, 16 (Figure 1a–c). It has been shown that such polymerisation can be blocked by competition with synthetic P14-3 or P7-2 peptides14, 17, 18, 19 (Table 1), but peptides are unsuitable agents for long-term therapy and both peptides are too large to act as a basis for mimetic drug design. Moreover, the results with different peptides seemed to be unpredictable and often surprising, with the P14-3 peptide from antithrombin acting as a better blocking agent with antitrypsin than does antitrypsin's own homologous peptide. Another indication of the unpredictability of binding came from the crystallographic study by Xue et al.,20 with the serpin plasminogen activator inhibitor, PAI-1, where a P14-10 peptide used to block inhibitory activity was unexpectedly found to also bind to the P6-2 position at the bottom of the A-sheet.
The explanation of these diverse findings developed from an observation at the commencement of this study; that a tetrapeptide WMDF (Trp-Met-Asp-Phe), unrelated to the serpins, was found to effectively block the polymerisation of both antitrypsin and antithrombin (Figure 1d). This control tetrapeptide, derived from cholecystokinin, provided a basis for the selection, by screening, of several relatively effective tetra- and tri-peptide blocking agents. The determination of the crystal structures of these small blocking peptides bound to antithrombin clarifies the specific requirements for peptide binding and also provides a structural explanation of the previously puzzling observations with natural peptides. Definition of the structural requirements for peptide binding has now enabled the elective design of a novel tetrapeptide that selectively blocks the polymerisation of the conformationally unstable variants of antitrypsin. This is demonstrated here with a priority medical target, the Z mutant of antitrypsin, commonly present in people of European descent, which causes a predisposition to emphysema and liver cirrhosis.6
Section snippets
Peptide assessment
Altogether some 40 small peptides of three to five amino acid residues randomly selected from commercial sources, together with peptides derived from serpin reactive loop sequences were screened as potential blocking agents, using different polymerogenic test-serpins. Antithrombin is converted to a polymerogenic form with the A-sheet opened by prior annealing of a P14-9 (or P14-8) peptide to the upper half of the strand 4 position.18 As shown in Figure 2a, antithrombin readily forms polymers in
Discussion
The conformational diseases such as Alzheimer's, Parkinson's and the prion encephalopathies pose a special challenge to structural biologists. They are all diseases resulting from aberrant β-strand interlinkages. Yet the end aim of research into the underlying molecular mechanisms, to develop specific therapies to prevent these intermolecular linkages, seems to be a daunting prospect. However, the insights we now have from the prototypical conformational diseases of the serpins are much more
Proteins
Human antithrombin and antitrypsin were isolated from fresh frozen human plasma as previously described.8, 29 PP4, a glycine specific cysteine protease that cleaves antitrypsin at P1021, was purchased from CN Biosciences UK, Nottingham, UK.
Peptide selection
Altogether some 40 small peptides of three to five amino acid residues were randomly selected from commercial sources and various serpin derived peptides were synthesised. The synthetic peptides Ac-SEAAASTAVVIA (antithrombin P14-3), Ac-SEAAAST (antithrombin
Acknowledgements
This study was supported by the Wellcome Trust, the NIH, the MRC, the Isaac Newton Trust, and the Alpha-1 Foundation. We thank Dr Helen Parfrey, University of Cambridge, for providing Z antitrypsin and Professor Randy J. Read for advice on crystallography.
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