Schnell D J, Etzler M E
J Biol Chem. 1987 May 25;262(15):7220-5.
The Dolichos biflorus seed lectin is a tetramer composed of equal amounts of two subunit types. The subunit types are structurally very similar, yet only the larger subunit exhibits the ability to bind carbohydrate. A cDNA clone representing the entire coding region of the D. biflorus lectin mRNA has been sequenced. This cDNA represents 1075 nucleotides of seed lectin mRNA encoding a polypeptide of Mr = 29,674. Analysis of the deduced sequence indicates that the NH2 termini and COOH termini of both lectin subunits are present within the mRNA coding region. This information supports previous data indicating that both subunits of the lectin are encoded by a single mRNA and that the difference between the subunit types apparently arises by the proteolytic removal of a 10-amino acid sequence from the COOH terminus of the larger subunit. Comparison of the D. biflorus seed lectin sequence to the sequence of other leguminous seed lectins indicates regions of extensive homology. The residues of concanavalin A involved in metal binding are highly conserved in the D. biflorus lectin, but those involved in saccharide binding show a much lower degree of conservation. Prediction of the secondary conformation of the D. biflorus polypeptide suggests that structures involved in the formation of quaternary structure in concanavalin A are also conserved.
双花扁豆种子凝集素是一种四聚体，由等量的两种亚基类型组成。这两种亚基类型在结构上非常相似，但只有较大的亚基具有结合碳水化合物的能力。一个代表双花扁豆凝集素mRNA完整编码区的cDNA克隆已被测序。这个cDNA代表了1075个核苷酸的种子凝集素mRNA，编码一个Mr = 29,674的多肽。对推导序列的分析表明，两种凝集素亚基的NH2末端和COOH末端都存在于mRNA编码区内。这一信息支持了先前的数据，即凝集素的两个亚基都由一个单一的mRNA编码，并且亚基类型之间的差异显然是由于从较大亚基的COOH末端蛋白水解去除了一个10个氨基酸的序列而产生的。将双花扁豆种子凝集素序列与其他豆科种子凝集素的序列进行比较，发现存在广泛的同源区域。伴刀豆球蛋白A中参与金属结合的残基在双花扁豆凝集素中高度保守，但参与糖类结合的残基保守程度要低得多。对双花扁豆多肽二级构象的预测表明，伴刀豆球蛋白A中参与四级结构形成的结构也具有保守性。
