2 - Structure of lectins

Summary

Most of our knowledge of the primary structure of plant lectins has been gathered in the last decade from extensive chemical studies with lectins isolated from leguminous plants. The comparative, evolutionary and chemotaxonomical aspects of such studies are of particularly special value for our understanding of the potential functional significance of lectins and gene alterations in plants during evolution.

Primary sequences

A comparison of partial or complete primary sequences of lectins of diverse origin, including one- or two-chain lectins, has shown up extensive homologies between different lectins (Strosberg et al., 1986a,b). Thus, for example, the amino terminal sequences of the β-chains of two-chain lectins from Vicieae and of one-chain lectins from other tribes of Papilionaceae or Caesalpinoideae, compared with the appropriate sequence of concanavalin A (residues of 123–147), are highly homologous (Strosberg et al., 1983; 1986a,b). It has also become clear from sequence comparisons that lectins from the same tribe are more homologous to each other than to members of other tribes. Rather interestingly, phenylalanine residues in positions 6 and 11 are conserved in all 26 lectins used for comparison studies. Residue 5 is also found to be highly conserved and occurs in 23 of the 26 lectins studied.

Thus, the comparison of even partial sequences has turned out to be very rewarding. However, more is to be expected from comparisons of the structure of fully sequenced lectins, and the value of such structural comparisons is likely to grow in future. Up until now, Strosberg and his colleagues have already compared the full primary amino acid sequences of seven leguminous lectins.