Comparative Biochemistry of the Galectin Families

 Galectins comprise a family of animal lectins that bind to -galactoside-containing carbohydrate moieties of glycoconjugates (See LE-A01 “Galectin: Definition and History” in “Lectins”). Galectins have been identified in various animal species including nematode, conger eel, fly, zebrafish, rainbow trout, chicken, etc. In mammals, currently fourteen galectins have been isolated and they are classified into three subgroups according to their domain structures (galectins-1-14). We have introduced Xenopus laevis as a model system for studying the roles of galectins, and Xenopus is the only vertebrate whose galectin family has been comprehensively identified, other than mammals (ref. 1, 2; Xenopus galectin: xgalectins-Ia-VIIIa). In this issue, the author compares the Xenopus and mammalian galectin families.
Fig1.Members of the Xenopus and mammalian galectin families
Galectins in the green circle are apparent homologues between mammals and Xenopus. Small a and b in the names of Xenopus galectins indicate isoforms whose amino acid sequences are highly similar.

According to the similarities of amino acid sequences and their expression patterns, Xenopus and mammalian galectins can be categorized into two groups in which possible homologous relationships could or could not be determined (Fig. 1). Xgalectin-VIIa and mammalian galectin-3 were determined to be homologous and we have also observed similar specificity in their oligosaccharide recognition. A representative pair with a similar expression pattern was found when comparing the expressions of xgalectins-IIa & IIb and galectin-4. Their mRNAs were all localized in digestive tracts. However, the amino acid identity of every pair was limited to around 50 %. This value is lower than in the case of sequence comparison of factors that are very essential for an organism, such as some transcription factors, growth factors and so on. In general, we are able to obtain identity of over 70% in the sequence comparison of these factors, whose functions seem to be basic and universal, even between homologues of frog and mammals.

On the other hand, there are a number of Xenopus and mammalian galectins whose homologous relationships cannot be estimated. One representative example is skin-galectins. In both Xenopus and mammals, there are galectins that are specifically and abundantly expressed in skin. These are xgalectin-Va &Vb and mammalian galectin-7. However, we cannot estimate their homologous relationship from their amino acid sequence similarity because the identity of sequences of xgalectin-Va and galectin-7 is only 15%. This fact reminds us that there are great differences in the skin of Xenopus and mammals. We do not know if the difference in skin-specific galectin is a result or a cause of the difference in skin, but it is quite likely that the skin-specific galectins are related to the characterization of the skin of each species. Taking into consideration, the fact that similarities in homologous galectins between species are relatively low, galectins seem to have not only universal functions across species, but also seem to contribute greatly to the characterization of each species. In fact, the galectin-7 gene is still found only in mammals, even though DNA databases for various species have been compiled (3). Another piece of evidence is found in the expression pattern of Xenopus xgalectin-VIa, whose mammalian counterpart has not been identified. During embryogenesis, the distribution of mRNA of xgalectin-VIa is restricted to the cement gland, which is a transient organ specific to amphibians (Fig. 2).
Fig2. Distribution of mRNA of xgalectin-VIa was analyzed by whole-mount in situ hybridization.
The mRNA was localized in the cement gland,which is indicated by an arrow.

The author believes that most oligosaccharide-related genes and oligosaccharides themselves participate in the characterization of each species. Moreover, they may also be one of the great motivating forces behind evolution.
Hiroki Shoji (Faculty of Medicine, Kagawa University)
References (1) Shoji H, Nishi N, Hirashima M, Nakamura, T: Purification and cDNA cloning of Xenopus liver galectins and their expression. Glycobiology, 12, 163-172, 2002
(2) Shoji H, Nishi N, Hirashima M, Nakamura T: Characterization of the Xenopus galectin family. Three structurally different types as in mammals and regulated expression during embryogenesis. J. Bio.l Chem., 278, 12285-12293, 2003
(3) Houzelstein D, Goncalves IR, Fadden AJ, Sidhu SS, Cooper DN, Drickamer K, Leffler H, Poirier F: Phylogenetic analysis of the vertebrate galectin family. Mol. Biol. Evol., 21, 1177-1187, 2004
Jan. 24, 2005

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