This proposal addresses continuing efforts to elucidate the structure of the lectin isolated from the garden pea (PSA) at high resolution by protein crystallographic methods. The objectives are to refine the model structure, based on a 3 angstrom electron density map, by restrained least-squares methods to an ultimate resolution of 1.2 angstroms. PSA is a 25 K dalton protein composed of two polypeptides of ca. 7K and 18K daltons, requires Mn++ and Ca++ for carbohydrate-binding activity and crystallizes as a dimer of 50K daltons. The diffracting quality of the PSA crystals offer a unique opportunity to examine this relatively large protein at high resolution (1.2 angstroms). High-resolution data will be collected on the unique facility at Daresbury, England consisting of a Arndt/Nonius T.V. detection system, synchrotron wiggler beam line and 1.0 angstrom wavelength radiation. The structure of PSA will be carefully compared with the structure of Con A in order to understand the structural implications of the unusual circularly permuted amino acid sequence compared to Con A. The analysis of similarities between the two subunits that make up the asymmetric unit in PSA crystals, Con A and a predicted structure of PSA will help in our understanding of the effect of amino acid sequences on the tertiary and quaternary structures of proteins. The details of the carbohydrate-binding site probed with both simple and complex carbohydrates will be determined by crystallographic studies of the PSA-carbohydrate complexes. These efforts will be extended to other lectins closely related to PSA by model building using computer graphics. The structure of lectin-carbohydrate complexes will help in our understanding of the specificities involved in carbohydrate-protein interactions and the recognition of carbohydrates, glycoproteins and cell surface components by proteins.