COMPARISON OF CRYSTAL-STRUCTURES OF 2 HOMOLOGOUS PROTEINS - STRUCTURAL ORIGIN OF ALTERED DOMAIN INTERACTIONS IMMUNOGLOBULIN LIGHT-CHAIN DIMERS

The sequence and structure of a second human kappa(I) immunoglobulin l ight-chain variable domain, Wat, has been determined. The R-factor is 15.7% for 1.9-Angstrom data. One hundred and ninety-five water molecul es were identified; 30 water molecules were located in identical posit ions in each of the monomers. Some of the water molecules are integral parts of the domains. This light chain is encoded by the same variabl e domain gene that encoded the previously characterized kappa(I) varia ble domain, Rei. Due to limited somatic mutation, the two highly homol ogous proteins differ in only 20 of the 108 residues. Wat crystallized in space group P6(4) while Rei crystallized in space group P6(1); in both crystals, the asymmetric unit was the noncovalent dimer. Although the basic domain structure is the same for both proteins, the relativ e positions of the domains within the two dimers differ. This differen ce is most likely accounted for by the replacement of Tyr36 in Rei by Phe in the Wat protein. Residue Tyr36 is part of the hydrogen-bonding network in the interface between the domains in Rei. Losing the hydrog en-bonding capability of residue 36 by replacement of Tyr by Phe alter s the network of hydrogen bonds between the domains, resulting in a di fferent domain-domain contact. The details of lattice contacts in the two crystals were compared. One type of contact that extends the beta- sheet of the individual domains was conserved, but because it involved different symmetry elements within the crystal, different crystal pac king resulted. In the Wat crystal, one of the contacts shows an exampl e of how a symmetrical binding site can ''bind'' an asymmetrical objec t. Further, the examination of the Wat crystal also illustrates how th e different crystalline environments of the domains of the dimer resul ts in different distributions of temperature factors for the residues within the domains.