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The complete primary structure of a galactose-specific lectin contained in the
venom of the rattlesnake, Crotalus atrox, was determined. The lectin is composed
of two covalently linked, identical subunits, each consisting of 135 amino acid
residues. Under physiological conditions the lectin proved to be highly
aggregated. The venom lectin contained 9 half-cystines, 8 of which formed four
intrasubunit disulfide bridges (Cys3-Cys14, Cys31-Cys131, Cys38-Cys133, and
Cys106-Cys123), while Cys86 was involved in an intersubunit disulfide bridge.
Because of the high content of disulfide bridges, the intact lectin was
extremely resistant to tryptic digestion. The determined amino acid sequence was
found to be homologous with those of the so-called carbohydrate recognition
domains of Ca2(+)-dependent-type lectins in animal. Among them, 8 amino acid
residues (Cys31, Gly69, Trp92, Pro97, Cys106, Asp120, Cys123, and Cys131) were
completely conserved. Leu40, Trp67, and Trp81 were also well conserved. The
rattlesnake venom lectin showed high hemagglutinating activity. These results,
together with the occurrence of similar lectins in crotalid venoms, suggest that
these lectins have evolved in order to make the venom a more effective weapon to
capture prey animals.
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