Sequence variability in three wheat germ agglutinin isolectins: products of multiple genes in polyploid wheat

Three highly homologous wheat germ isolectins (95-97%) are distinct gene products in hexaploid wheat. The amino acid sequences of two of these [wheat germ agglutinin 1 (WGA1) and 2 (WGA2)] are compared with sequence data derived from a complementary DNA (cDNA) clone for the third isolectin (WGA3). This comparison includes three corrections to earlier amino acid sequence data of both WGA1 and WGA2 at positions 109 (from Ser to Phe), 134 (from Gly to Lys), and 150 (from Gly to Trp). These reassignments are based on new results from crystal structure refinement and amino acid sequence data of WGA1, as well as the recently determined nucleotide sequence of WGA3. In addition, the C-terminal residue of WGA1 has been revised to Gly171 and now differs from WGA2 (Ala171). Four other positions, Asn9, Ala53, Gly119, and Ser123, at which WGA1 and WGA2 are identical but differ from the DNA sequence of WGA3, were also reinvestigated by amino acid sequencing techniques and confirmed. Variability among the three isolectins is observed at a total of 10 sequence positions: 9, 53, 56, 59, 66, 93, 109, 119, 123, and 171. Pairwise comparisons indicate that WGA3 deviates to a much larger extent from WGA1 (at eight positions) and from WGA2 (at seven positions) than the latter from one another (at five positions). Eight of the 10 mutations are equally distributed between domains B and C, the two interior and more highly conserved of the four WGA domains (A, B, C, D). Correlation of the variable residues with the three-dimensional structure indicates that all except the two previously described B-domain residues, 56 and 59 (Wright and Olafsdottir 1986), are easily accommodated at the dimer surface. WGA3 displays a higher degree of inter-domain similarity than found in WGA1 and WGA2. Of the seven variable positions that are located in the domain core (residues 3-31), five are in perfect agreement with our earlier predicted domain ancestor sequence. This suggests that of the three isolectins WGA3 is most closely related to the common ancestral molecule.