STRUCTURE OF A TYPE III AFP PHASED BY THE ANOMALOUS SIGNAL OF A SINGLE IODINE ATOM USING CUK[[alpha]] RADIATION. ¹Yang, D.S.C., ¹Bubanko, S., ¹Xue, Y.Q., ¹Seetharaman, J., ²Hew, C.L., ³Fletcher, G.L. and ¹Sicheri, F.. ¹Department of Biochemistry, McMaster Univeristy, Hamilton, Ontario L8N 3Z5 Canada,; ²Res. Inst., Hosp. for Sick Children, Toronto, Ontario M5G 1X8, Canada; ³Dept. Ctr., Memorial University of Newfoundland, St.Johns, NFLD A1C 5C7 Canada

Four distinct types of antifreeze polypeptides (AFPs) have been isolated from polar marine fish, all of which act to inhibit ice growth through direct adsorption to the ice lattice. The a-helical structure of type I AFP has been solved and its mechanism is currently under investigation. A solution structure of a type III AFP has been determined by 2D NMR spectroscopy. It differs from type I AFP in that this structure consists of two sheets of three antiparallel strands and one sheet of two antiparallel strands; with the triple-stranded sheets forming a ß-sandwich.

We have crystallized type III AFPs in three different crystal forms. Extensive molecular replacement (MR) studies using the NMR derived structure failed to yield a solution. We have recently crystallized an iodotyrosine derivatized AFP in a fourth crystal form. The Iterative Single Anomalous Scattering (ISAS) procedure was applied to determine the structure. Anomalous signal from four iodine atoms and four fold noncrystallographic symmetry averaging were used successfully to phase the structure which consists of 280 residues in the asymmetric unit. Details of data collection, processing and structure refinement will be presented along with a discussion of the MR effort.