HOW TO ESCAPE FROM MODEL BIAS WITH A HIGH-RESOLUTION NATIVE DATA SET - STRUCTURE DETERMINATION OF THE PCPA-S6 SUBUNIT-III

The structure of procarboxypeptidase A-S6 subunit III, a truncated zym ogen E, has been determined by molecular replacement using as search m odel porcine elastase 1 which, as revealed by crystallographic analysi s, contained about 20% of the amino acids in a radically different ori entation. Two monoclinic crystal forms were used: the first one diffra cts to 2.3 Angstrom resolution and contains one molecule per asymmetri c unit; the second diffracts to 1.7 Angstrom resolution and contains t wo molecules per asymmetric unit. Molecular replacement and convention al X-PLOR refinement led to a model for which 20% of the chain was ill defined in both crystal forms. To remove the bias introduced by the i nitial model, an automated refinement procedure [Lamzin & Wilson (1993 ). Acta Cryst. D49, 129-147] was applied successfully to the second cr ystal form, which diffracts to high resolution. The resulting dramatic improvement of the electron-density map led to extensive rebuilding o f some surface loops. The reliability of the modified model was confir med by refinement of the first crystal form. For the two forms, the fi nal R factor is 18.8% for data between 8.0 and 2.0 Angstrom resolution , and 18.4% for data between 8.0 and 1.7 Angstrom, respectively.