Toward the Crystal Structure of a Type III Antifreeze Protein From Ocean Pout, Macrozoarces Americanus

Abstract

<p> Four stucturally distinct types of macromolecular antifreezes have been previously isolated from the sera of polar marine fish. When the water temperature surrounding these organisms drops below -0.7°C, the freezing point of their bodily fluids, any contact with surrounding ice will nucleate internal ice crystal growth. The antifreeze proteins (AFPs) and antifreeze glycoproteins (AFGPs) synthesized by the fish act to inhibit the growth of existing ice crystals in their sera through direct adsorption to the ice lattice. The α-helical structure of type I AFP from winter flounder has been solved to atomic resolution and its mechanism of ice binding has been proposed. The NMR solution structure of a type III AFP from ocean pout has identified proteins in this class to exist in a β-sandwich conformation, however their mechanism of action remains uncertain.</p> <p> To facilitate the pursuit of an x-ray crystal structure solution, we subcloned the gene for a type III AFP (HPLC6) into pET15b and expressed recombinant His-rHPLC6 AFP in E. coli. Purified rmHPLC6 product has been successfully crystallized, and heavy atom soaks were performed in order to attempt a structure solution by multiple isomorphous replacement. The lone tyrosine in this recombinant AFP has been successfully derivatized in solution with iodine, and the modified protein was crystallized. In order to optimize the measurement of anomalous scattering information, modifications to our data collection system were required. Cryocrystallography techniques were employed to improve the quality of collected data.</p> <p> The expression, purification, crystallization and optimized data collection on an iodine-derivatized type III AFP from ocean pout will be presented here. This work has been instrumental in providing the high quality x-ray data required to solve the crystal structure to atomic resolution. Future examination of the solved structure will promote an increased understanding of the ice-binding mechanism exhibited by this class of proteins.</p>