W0081: Indication of Model-Bias at Atomic Resolution: Experimental Phasing of An Ef-Hand Protein at 1.05Å

W0081

Indication of Model-Bias at Atomic Resolution: Experimental Phasing of an EF-hand Protein at 1.05Å. D. E. Brodersen^*, E. de La Fortelle^[dagger], G. Bricogne^[dagger], J. Nyborg^*, and M. Kjeldgaard^*, ^*Macromolecular Crystallography, IMSB, Aarhus University, Gustav Wieds Vej 10c, DK-8000 Aarhus C, Denmark, ^[dagger]MRC-LMB, Hills Road, Cambridge, CB2 2QH, England

An experimental 1.05Å electron-density map, not biased byany atomic model, has been obtained by using single wavelength anomalous dispersion (SAD) data collected for the lanthanide substituted S100 calcium-binding protein psoriasin [1] at a wavelength of 0.9091Å (i.e. far from anyabsorption edges, yet with significant anomalous signal). These data alone were used to refine the heavy atom parameters and calculate phase probability distributions in the statistical phasing program SHARP [2, 3]. In a second step, 130 cycles of solvent flattening were applied in SOLOMON [4] using a special procedure implemented in the SHARP interface that slowly decreases the radius of the `solvent sphere' from an initial value of 3Å until it reaches 1.05Å. The effect of this procedure is to create a mask that starts as a molecular envelope and slowly becomes as a `glove' around the density of each residue. The resulting electron density map is of outstanding quality and offers a unique possibility to gain insight into high-resolution features of proteins such as local disorder, conformational variations, thermal motion, and preferred geometry. Moreover, the data indicate that significant bias towards model phases is likely to occur during refinement of macromolecular structures even at high- to atomic resolution. Traditional conjugate gradient LS refinement including anisotropy and riding hydrogens was done in SHELX and carried out with the same SAD data and separate Friedel mates.

1. Brodersen, D. E., et al, and M. Kjeldgaard. (1998) Structure, in press.

2. de La Fortelle, E., Irwin, J.J., Bricogne, G. Crystallographic Computing, 7.

3. de La Fortelle, E. and Bricogne, G. (1997) Methods in Enzymology, 276, 472-494.

4. Abrahams, J. P. and Leslie, A. G. W. (1996) Acta Cryst., D52: p. 30-42.