| The l.5A x-ray crystal structure of a lone {dollar}alpha{dollar}-helical antifreeze protein (AFP) from winter flounder is presented, providing the first detailed look at an AFP and its ice binding features. The structure was determined using a nonconventional multi-parameter molecular replacement scheme with two idealized {dollar}alpha{dollar}-helices as the search models. The described method may prove useful for the determination of other {dollar}alpha{dollar}-helical structures. The AFP's ice binding structure consists of four repeated ice binding motifs (IBMs), the side chains of which are inherently rigid or restrained by pair-wise side chain interactions to form a flat binding surface. Comparison shows that analogous IBM sequences are found highly conserved in all members of the {dollar}alpha{dollar}-helical class of AFPs. Also revealed are the presence of elaborate N- and C-terminal cap structures which help to account for the AFP's unusual stability for a lone {dollar}alpha{dollar}-helix in aqueous solution. Based on the crystal structure, a model amenable to testing is proposed which accounts for the proteins ice binding ability and specificity. Experiments to test the validity and generality of this ice binding model are proposed. |
