Primary structure of the Dolichos biflorus seed lectin

Area of Science:

• Plant molecular biology
• Biochemistry
• Structural biology

Background:

• Dolichos biflorus seed lectin is a tetramer with two subunit types.
• Only the larger subunit binds carbohydrates, despite structural similarity.
• Previous data suggest both subunits originate from a single mRNA.

Purpose of the Study:

• To sequence the cDNA clone of the D. biflorus lectin mRNA.
• To analyze the deduced amino acid sequence and compare it to other lectins.
• To understand the structural basis for subunit differences and conserved features.

Main Methods:

• Sequencing of a cDNA clone encoding the D. biflorus lectin mRNA.
• Deduction of the polypeptide sequence and analysis of termini.
• Bioinformatic comparison with other leguminous seed lectins, including Concanavalin A.

Main Results:

• The cDNA represents 1075 nucleotides, encoding a 29,674 Mr polypeptide.
• Both NH2 and COOH termini are within the mRNA coding region.
• Subunit differences arise from proteolytic removal of a 10-amino acid sequence from the larger subunit's COOH terminus.
• Homology observed with other leguminous lectins; conserved metal-binding residues, but less conserved saccharide-binding residues.
• Conserved structures for quaternary structure formation, similar to Concanavalin A.

Conclusions:

• D. biflorus seed lectin subunits are derived from a single mRNA via post-translational modification.
• Structural conservation exists in metal binding and quaternary structure formation sites, but not significantly in carbohydrate binding sites.
• Insights into lectin evolution and structure-function relationships within the Leguminosae family.