Interaction of external protein ligands with sugar bearing membrane receptors are known to trigger many cellular processes. This proposal addresses the specific question of the stereochemical requirements that lead to productive protein-cell surface receptor complex formation. Lectin-sugar complexes have served as useful biological model systems to answer this question. Such answers require solid structural knowledge of both lectin and receptor on the atomic level in order to be definitive. Wheat germ agglutinin (WGA) is one of the most thoroughly studied lectins and known to specifically interact with sialic acid bearing receptor glycoproteins on eukaryotic cell membranes, thereby causing numerous effects such as cytotoxicity or morphological restriction. Two closely homologous molecular forms of this lectin (isolectins 1 and 2) are being studied in atomic detail by x-ray crystallographic and chemical techniques. Knowledge obtained from the recently determined high resolution structure (1.8A) of isolectin 2 will be used in continuing studies to refine the crystal structure of isolectin 1 at 2.0A resolution and to determine the structure of the isolectin 1/N-acetyl neuraminyl lactose complex at 2.2A. The structure of this complex will subsequently be compared with the analogous one of isolectin 2 (2.2A) to explain their differential saccharide binding affinities. We also propose to determine the crystal structure of a compl x of WGA with a sialoglycopeptide (T-5) derived from the major red cell membrane protein glycophorin A, a specific receptor for WGA. This will be done by a combination of the methods of heavy atom isomorphous replacement and molecular replacement. Using the refined WGA structure it will be possible to analyze in detail the stereochemistry of this complex and determine the extent to which carbohydrate and peptide interact with the lectin surface. The structure of this complex will be a more definitive model for protein-cell surface saccharide interactions than are the simple saccharide/WGA complexes explored up to now.