A 2.6 angstrom structure of a serpin polymer and implications for conformational disease

The function of the serpins as proteinase inhibitors depends on their abili ty to insert the cleaved reactive centre loop as the fourth strand in the m ain A beta-sheet of the molecule upon proteolytic attack at the reactive ce ntre, P1-P1'. This mechanism is vulnerable to mutations which result in ina ppropriate intra- or intermolecular loop insertion in the absence of cleava ge. Intermolecular loop insertion is known as serpin polymerisation and res ults in a variety of diseases, most notably liver cirrhosis resulting from mutations of the prototypical serpin alpha(1)-antitrypsin. We present here the 2.6 Angstrom structure of a polymer of alpha(1)-antitrypsin cleaved six residues N-terminal to the reactive centre, P7-P6 (Phe352-Leu353). After s elf insertion of P14 to P7, intermolecular Linkage is affected by insertion of the P6-P3 residues of one molecule into the partially occupied beta-she et A of another. This results in an infinite, linear polymer which propagat es in the crystal along a 2-fold screw axis. These findings provide a frame work for understanding the uncleaved alpha(1)-antitrypsin polymer and fibri llar and amyloid deposition of proteins seen in other conformational diseas es, with the ordered array of polymers in the crystal resulting from slow a ccretion of the cleaved serpin over the period of a year. (C) 1999 Academic Press.