CRYSTAL-STRUCTURE OF SER-22 ILE-25 FORM CRAMBIN CONFIRMS SOLVENT, SIDE-CHAIN SUBSTATE CORRELATIONS/

It is not agreed that correlated positions of disordered protein side chains (substate correlations) can be deduced from diffraction data, T he pure Ser-22/Ile-25 (SI form) crambin crystal structure confirms cor relations deduced for the natural, mixed sequence form of crambin crys tals, Physical separation of the mixed form into pure SI form and Pro- 22/Leu-25 (PL form) crambin and the PL form crystal structure determin ation (Yamano, A., and Teeter, M. M. (1994) J. Biol, Chem, 269, 13956- 13965) support the proposed (Teeter, M. M., Poe, S. IM., and Heo, N. H . (1993) J. Mol. Biol, 230, 292-311) correlation model, Electron densi ty of mixed form crambin crystals shows four possible pairs of side ch ain conformations for heterogeneous residue 22 and nearby Tyr-29 (2(2) = 4, two conformations for each of two side chains), One combination can be eliminated because of short van der Waals' contacts, However, o nly two alternates have been postulated to exist in mixed form crambin : Pro22/Try-29A and Ser-22Try-29B. In crystals of the PL form, Pro-22 and Tyr-29A are found to be in direct van der Waals' contact (Yamano, A., and Teeter, M. M. (1994) J. Biol, Chem, 269, 13956-13965), Compari son of the SI form structure with the mixed form electron density conf irms that the fourth combination of side chains does not occur and tha t side chain correlations are mediated by water networks.