Orthorhombic crystals of isolectin I (LOLI) from the seeds of Lathyrus ochrus were first obtained during the STS 29 space shuttle mission. Subsequently, isostructural crystals were also obtained in the laboratory. They belong to the space group P21212, with cell dimensions a = 135.84 Å, b = 63.12 Å and c = 54.54 Å with one functional entity, a dimer, in the asymmetric unit (Vm = 2.2 Å3/Da). The three-dimensional structure of LOLI, which was solved by the molecular replacement method using a 3 Åresolution model of pea lectin, has subsequently been refined by using crystallographic data between 8.0 Åand 1.9 Åresolution, coupled to molecular dynamics and energy minimization techniques. The conventional R-factor is 0.185 for F01σ(F0). The final model includes 220 well-defined water molecules and has root-mean-square deviations from ideal bond distances and angles of 0.004 Åand 3°, respectively. Only slight conformational differences have been found between the two αβ monomers. The molecular structure of LOLI, the first to be determined from the genus Lathyrus, is very similar to those of concanavalin A, pea lectin and favin. Differences are confined to the loop regions and β-chain termini. Comparison of equivalent Cα atom positions between our final model and the pea lectin structure shows slight differences in the association of the two monomers, which are probably due to the different environments in the crystals. The root-mean-square deviation between Cα atoms of LOLI and pea lectin is 0.40 Å. The metal binding sites are very similar in pea lectin, concanavalin A and LOLI. The sugar-binding sites of LOLI are occupied by four well-ordered water molecules each. The cleavage site for one of the monomers is specially well defined in the final electron density map: the amino group of Glul (α) seems to form a salt bridge with the car{ballot box}ylate group of the terminal Asn181 (β). A detailed analysis of the difference in crystal packing contacts between pea lectin and LOLI shows that, as might be expected, several of the intermolecular interactions are mediated by residues that correspond to substitutions in the LOLI amino acid sequence. © 1990 Academic Press Limited.
