THE CRYSTALLOGRAPHIC STRUCTURE OF PHYTOHEMAGGLUTININ-L

The structure of phytohemagglutinin-L (PHA-L), a leucoagglutinating se ed lectin from Phaseolus vulgaris, has been solved with molecular repl acement using the coordinates of lentil lectin as model, and refined a t a resolution of 2.8 Angstrom. The final R-factor of the structure is 20.0%. The quaternary structure of the PHA-L tetramer differs from th e structures of the concanavalin A and peanut lectin tetramers, but re sembles the structure of the soybean agglutinin tetramer. PHA-L consis ts of two canonical legume lectin dimers that pack together through th e formation of a close contact between two beta-strands. Of the two co valently bound oligosaccharides per monomer, only one GlcNAc residue p er monomer is visible in the electron density. In this article we desc ribe the structure of PHA-L, and we discuss the putative position of t he high affinity adenine binding site present in a number of legume le ctins. A comparison with transthyretin, a protein that shows a remarka ble resemblance to PHA-L, gives further ground to our proposal.