Characterization of a Soybean Leaf Protein That Is Related to the Seed Lectin and Is Increased with Pod Removal

Abstract

Levels of several polypeptides in addition to the vegetative storage protein (VSP) increase in soybean leaves following depodding. Two of these polypeptides interact specifically with antibodies raised against the seed lectins of Phaseolus vulgaris and soybean. The two polypeptides, which had apparent molecular masses of 29,000 daltons and 33,000 daltons, were present in the sink-deprived plants but not in control podded plants and were the subunit polypeptides of a glycoprotein designated lectin-related protein (LRP). Soybean LRP was purified to near homogeneity by a combination of ammonium sulfate precipitation and gel filtration. Dialysis of the resuspended ammonium sulfate precipitate caused LRP to reprecipitate, and LRP was soluble only in the presence of molar NaCl. The native relative molecular mass of LRP was 119,000 daltons, a size consistent with a tetrameric organization of the two polypeptides. LRP precipitated during dialysis in association with a 28,000 dalton polypeptide. The protein coprecipitating with LRP was identified as the dimer of the 28,000 dalton subunit of VSP, one of three native isomeric forms of VSP occurring in leaves of depodded plants. Although the specific association between LRP and VSP was intriguing, an in vivo interaction between LRP and VSP was doubtful. LRP was shown to be immunologically similar to soybean agglutinin but did not have detectable hemagglutinating activity. LRP also was shown to be made up of polypeptides distinct from soybean agglutinin.