CRYSTAL-STRUCTURE OF THE ARCELIN-1 DIMER FROM PHASEOLUS-VULGARIS AT 1.9-ANGSTROM RESOLUTION

Arcelin-1 is a glycoprotein from kidney beans (Phaseolus vulgaris) whi ch displays insecticidal properties and protects the seeds from predat ion by larvae of various bruchids, This lectin-like protein is devoid of monosaccharide binding properties and belongs to the phytohemagglut inin protein family. The x-ray structure determination at 1.9-Angstrom resolution of native arcelin-1 dimers, which correspond to the functi onal state of the protein in solution, was solved using multiple isomo rphous replacement and refined to a crystallographic R factor of 0.208 , The three glycosylation sites on each monomer are all covalently mod ified. One of these oligosaccharide chains provides interactions with protein atoms at the dimer interface, and another one may act by preve nting the formation of higher oligomeric species in the arcelin varian ts. The dimeric structure and the severe alteration of the monosacchar ide binding site in arcelin-1 correlate with the hemagglutinating prop erties of the protein, which are unaffected by simple sugars and sugar derivatives. Sequence analysis and structure comparisons of arcelin-1 with the other insecticidal proteins from kidney beans, arcelin-5, an d alpha-amylase inhibitor and with legume lectins, yield insights into the molecular basis of the different biological functions of these pr oteins.