Bioproduction Research Institute, National Institute of Advanced Industrial Science and Technology, Sapporo 062-8517, Japan.
Proc Natl Acad Sci U S A. 2012 Jun 12;109(24):9360-5. doi: 10.1073/pnas.1121607109. Epub 2012 May 29.
Antifreeze proteins (AFPs) are found in organisms ranging from fish to bacteria, where they serve different functions to facilitate survival of their host. AFPs that protect freeze-intolerant fish and insects from internal ice growth bind to ice using a regular array of well-conserved residues/motifs. Less is known about the role of AFPs in freeze-tolerant species, which might be to beneficially alter the structure of ice in or around the host. Here we report the 0.95-Å high-resolution crystal structure of a 223-residue secreted AFP from the snow mold fungus Typhula ishikariensis. Its main structural element is an irregular β-helix with six loops of 18 or more residues that lies alongside an α-helix. β-Helices have independently evolved as AFPs on several occasions and seem ideally structured to bind to several planes of ice, including the basal plane. A novelty of the β-helical fold is the nonsequential arrangement of loops that places the N- and C termini inside the solenoid of β-helical coils. The ice-binding site (IBS), which could not be predicted from sequence or structure, was located by site-directed mutagenesis to the flattest surface of the protein. It is remarkable for its lack of regularity and its poor conservation in homologs from psychrophilic diatoms and bacteria and other fungi.
抗冻蛋白（AFPs）存在于从鱼类到细菌等生物体中，在这些生物体中，它们具有不同的功能，以促进宿主的生存。保护不耐冻的鱼类和昆虫免受内部冰生长的 AFP 通过规则排列的高度保守的残基/基序与冰结合。对于在耐冻物种中 AFP 的作用知之甚少，它可能是有益地改变宿主内或周围冰的结构。在这里，我们报告了来自雪霉真菌 Typhula ishikariensis 的 223 个残基分泌型 AFP 的 0.95 Å 高分辨率晶体结构。其主要结构元件是一个不规则的β-螺旋，带有六个 18 个以上残基的环，位于α-螺旋旁边。β-螺旋已经在几个场合作为 AFP 独立进化，似乎具有理想的结构，可以结合多个冰面，包括基面。β-螺旋折叠的新颖之处在于环的非顺序排列，将 N-和 C-末端置于β-螺旋线圈的螺线管内。冰结合位点（IBS）不能从序列或结构预测，通过定点突变将其定位到蛋白质最平坦的表面。它的特点是缺乏规律性，并且在来自嗜冷硅藻和细菌以及其他真菌的同源物中保守性差。
