Glycobiology Advance Access originally published online on January 31, 2006
Glycobiology 2006 16(5):402-414; doi:10.1093/glycob/cwj086
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The Geodia cydonium galectin exhibits prototype and chimera-type characteristics and a unique sequence polymorphism within its carbohydrate recognition domain
2 Institute of Biochemistry II, Medical Faculty, University of Cologne, Joseph-Stelzmann-Str. 52, 50931 Köln, Germany; 3 Central Bioanalytics, Center for Molecular Medicine Cologne (CMMC), University of Cologne, Köln, Germany; 4 Institute of Medical Physics and Biophysics, University of Münster, Münster, Germany; 5 Applied Biosystems, Langen, Germany; 6 Bruker-Daltonic, Bremen, Germany; and 7 Intradisciplinary Center for Biopolymers, University of Potsdam, Potsdam, Germany
1 To whom correspondence should be addressed; e-mail: franz.hanisch{at}uni-koeln.de
Received on October 25, 2005; revised on January 24, 2006; accepted on January 28, 2006
The ancestral galectin from the sponge Geodia cydonium (GCG) is classified on a structural basis to the prototype subfamily, whereas its carbohydrate-binding specificity is related to that of the mammalian chimera-type galectin-3. This dual coordination reveals GCG as a potential precursor of the later evolved galectin subfamilies, which is reflected in the primary structure of the protein. This study provides evidence that GCG is the LECT1 gene product, while neither a previously described LECT2 gene nor a functional LECT2 gene product was found in the specimen under investigation. The electrophoretically separated protein isomers with apparent molecular masses of 13, 15, and 16 kDa correspond to variants of the LECT1 protein-exhibiting peptide sequence polymorphisms that concern critical positions of the carbohydrate recognition domain (13 kDa: Leu51, Asn55, His130, Gly137; 15 kDa: Ser51, Asn55, Asn130, Gly137; 16 kDa: Ser51, Tyr55, Asn130, Glu137). Four residues, highly conserved in the galectin family, are substituted. None of the residues claimed to be involved in interactions with GalNAc
1-3 moieties at an extended binding subsite of galectin-3 was identified in the corresponding positions of GCG. Apparently, the substitutions do not confer distinct binding characteristics to the GCG variants as evidenced by binding studies with a recombinantly expressed 15-kDa isoform. The natural isoforms as well as the recombinant 15-kDa isoform oligomerize by the formation of non-covalent heteromeric or homomeric complexes. A phosphorylation of the galectin was confirmed neither by mass spectrometry nor by alkaline phosphatase treatment combined with isoelectric focusing.
Key words: galectins / mass spectrometry / polymorphism / sponge lectin