NMR STRUCTURES OF THE C-TERMINAL END OF HUMAN-COMPLEMENT SERINE-PROTEASE C1S

Synthetic peptides derived from the C-terminal end of the human comple ment serine protease Cls were analysed by circular dichroism and nucle ar magnetic resonance (NMR) spectroscopy. Circular dichroism indicates that peptides 656-673 and 653-673 are essentially unstructured in wat er and undergo a coil-to-helix transition in the presence of increasin g concentrations of trifluoroethanol. Two-dimensional NMR analyses per formed in water/trifluoroethanol solutions provide evidence for the oc currence of a regular x-helix extending from Trp659 to Ser668 (peptide 656-673), and from Tyr656 to Ser668 (peptide 653-673), the C-terminal segment of both peptides remaining unstructured under the conditions used. Based on these and other observations, we propose that the serin e protease domain of Cls ends in a 13-residue alpha-helix (656Tyr-Ser6 68) followed by a five-residue C-terminal extension. The latter appear s to be flexible and is probably locked within Cls through a salt brid ge involving Glu672.