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Iguanas in the Galapagos Islands, dive into the sea after waiting for their bodies to warm up from the rising sun. However, some fish (also poikilothermic) living in cold water, such as codfish, herring and sea raven, can be active even at below 0•Ž because they have a special "antifreeze protein" (AFP) in their sera which protects their bodies from freezing. Interestingly, a kind of AFP called "type II AFP" is a member of the Ctype lectin family [Ng, NFL., and Hew, C.L. (1992) J. Biol. Chem. 267, 16069-16075]. Though this AFP has no sugarbinding activity, it has the galactose-recognition motif, which is conserved between galactose-specific lectins in the C-type lectin family [picked up previously in this section; (1993) 5, 3335]. Ewart et al. (also the investigators of this AFP), probably being conscious of the original report [Drickamer, K. (1992) Nature 360, 183-186], introduced mutations to herring AFP (14,000 Da), which substitute galactose-recognition motif "Gln -Pro-Asp" with that for mannose recognition , "Glu-ProAsn". So, what was the result ? The antifreezing activity of this mutant protein was almost completely lost [Ewart, KY., Li, Z., Yang, D.S., Hletcher, G.L., and Hew, CL. (1998) Biochemistry 37, 4080-4085]. The relationship between lectin and ice is discussed. Up to the present, four types of AFPs as well as one antifreezing glycoprotein are known. These are thought to be evolutionarily non-related proteins. However, the mechanism by which they exert their functions can be the same: i.e., all of them inhibit ice crystal growth by interacting with the ice surface. Infact, a refined model has been proposed for both type-I and ‡V AFPs based on X-ray crystallography studies. On the other hand, a concrete mechanism has never been presented for type-‡U AFP. ‡T might add that pancreatic stone protein called lithotheotin, which inhibits crystallization of calcium carbonate in this tissue, is also a member of the C-type lectin. Since herring type-‡U AFP binds to calcium as other C-type lectins, the authors assumed that the calcium-binding site, which is in general responsible for carbohydrate recognition, is also involved in the interaction with ice crystals. To prove this, they constructed genes for the recombinant AFP having a His-Tag motif to facilitate purification, and further introduced mutagenesis changing the galactose-specific motif (Gln-Pro-Asp) occurring in herring AFP to a mannose-specific one (Glu-Pro-Asn). Thus expressed mutant AFP showed an almost identical CD specガ ラパ ゴ ス 島の イ グア ナ は朝 日を待 ちわ び て 海 に潜 る。変

Meaningful Relationship between C-type Lectin Galactose-binding Motif “Gln-Pro-Asp” and Antifreezing Activity