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A lectin was isolated from the homogenate of the tunicate Polyandrocarpa
misakiensis by heat treatment, ammonium sulfate fractionation, gel filtration,
and high-performance ion-exchange chromatography. Analytical gel filtration on
Superose 12 and sodium dodecyl sulfate-polyacrylamide gel electrophoresis showed
that the lectin is a monomeric protein with a molecular mass of approximately 15
kDa. The lectin bound to an immobilized D-galactose column in the presence of
calcium ion with a threshold of 500 microM and eluted completely with 5 mM EDTA.
It did not bind to an immobilized D-mannose or N-acetyl-D-galactosamine column.
Thus, Polyandrocarpa lectin was found to be a calcium-dependent
galactose-binding lectin. The complete amino acid sequence of Polyandrocarpa
lectin was determined by automated or manual Edman sequencing of the peptides
derived by digestion with trypsin, endoproteinase Asp-N, Staphylococcus aureus
V8 protease, and pepsin. It is composed of 125 residues, contains no
carbohydrate group, and has a calculated molecular mass of 14,034 Da. The lectin
contains four half-cystines, and Cys-21 and Cys-119 and also Cys-96 and Cys-111
form intrachain disulfide bridges, respectively. The amino acid sequence of
Polyandrocarpa lectin shows about 20-30% homology with those of fly, barnacle,
sea urchin, and several vertebrate lectins that belong to C-type lectin
(Drickamer, K. (1988) J. Biol. Chem. 263, 9557-9560). Although the physiological
role of Polyandrocarpa lectin is not clear, preliminary experiments suggest that
the lectin may be related to defense mechanisms because it has a strong
antibacterial activity.
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