The crystallographic structure of phytohemagglutinin-L

The structure of phytohemagglutinin-L (PHA-L), a leucoagglutinating seed lectin from Phaseolus vulgaris, has been solved with molecular replacement using the coordinates of lentil lectin as model, and refined at a resolution of 2.8 A. The final R-factor of the structure is 20.0%. The quaternary structure of the PHA-L tetramer differs from the structures of the concanavalin A and peanut lectin tetramers, but resembles the structure of the soybean agglutinin tetramer. PHA-L consists of two canonical legume lectin dimers that pack together through the formation of a close contact between two beta-strands. Of the two covalently bound oligosaccharides per monomer, only one GlcNAc residue per monomer is visible in the electron density. In this article we describe the structure of PHA-L, and we discuss the putative position of the high affinity adenine-binding site present in a number of legume lectins. A comparison with transthyretin, a protein that shows a remarkable resemblance to PHA-L, gives further ground to our proposal.