Reversible lattice repacking illustrates the temperature dependence of macromolecular interactions

Flash-freezing, which has become routine in macromolecular X-ray crystallog raphy, causes the crystal to contract substantially. In the case of Escheri chia coli beta -galactosidase the changes are reversible and are shown to b e due to lattice repacking. On cooling, the area of the protein surface inv olved in lattice contacts increases by 50%. There are substantial alteratio ns in intermolecular contacts, these changes being dominated by the long, p olar side-chains. For entropic reasons such side-chains, as well as surface solvent molecules, tend to be somewhat disordered at room temperature but can form extensive hydrogen-bonded networks on cooling. Low-temperature den sity measurements suggest that, at least in some cases, the beneficial effe ct of cryosolvents may be due to a density increase on vitrification which reduces the volume of bulk solvent that needs to be expelled from the cryst al. Analysis of beta -galactosidase and several other proteins suggests tha t both intramolecular and intermolecular contact interfaces can be perturbe d by cryocooling but that the changes tend to be more dramatic in the latte r case. The temperature-dependence of the intermolecular interactions sugge sts that caution may be necessary in interpreting protein-protein and prote in-nucleic acid interactions based on low-temperature crystal structures. ( C) 2001 Academic Press.