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ICCBM-5 meets in San Diego

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The 5th Int'l Conf. on the Crystallization of Biological Macromolecules was held in San Diego, CA, Aug. 8‑13, 1993. The meeting was organized by E.A. Stura, J.M. Sowadski, and J.E. Villafranca. The conference consisted of sessions on membrane protein crystallization, crystallization under microgravity, molecular biology as applied to crystallization problems, physics of crystallization, screening for crystallization conditions, and data collection facilities.

[Lumin Retinol-binding protein] Crystal of Lumin Retinol-Binding Protein. (Courtesy of J. Luft)
An NIH-sponsored Workshop on 'Practical Membrane Protein Crystallization' was conducted by H. Michel. P. Timmins presented evidence from neutron diffraction about how detergent molecules are organized in membrane-protein crystals. M. Garavito explained that new findings about the extreme sensitivity of membrane-protein crystals to their environmental conditions has resulted in many more examples of crystals being produced that are useful for data collection and heavy atom screening. W. Kuhlbrandt discussed special strategies appropriate for the production of two-dimensional crystals of membrane proteins required for electron diffraction work.

L. DeLucas described crystal growth experiments in microgravity and crystals of a number of proteins prepared during space shuttle missions, which exhibit significantly enhanced diffraction quality. Techniques and contributions to improved crystal quality described at the meeting included optimization of crystal growth in silica gels (A. McPherson and M.C. Roberts), systematic reductions of solvent content in crystals from levels in mother liquor (B. Schick), use of baculovirus, systems for protein expression (T.S. Gruenwald and B. McKinney), and the use of enzyme deglycosylation (H. Baker). Reports of efforts to monitor molecular interaction in solutions that are relevant to crystal growth included measurement of interaction potentials between protein molecules by small-angle scattering (A. Tardieu), steady-state fluorescence anisotropy (M. Jullien), neutron diffraction (M. Ataka), and quasielastic light scattering (QSEQ (R. Boistelle). A. Malkin and co-workers used QSEL to show that nucleation and crystal growth can be accelerated by adding latex microspheres, which act as heterogeneous nucleants, to the crystallization gemish. B. Wilson described the use of static light scattering to predict the most favorable conditions for protein crystallization. His results show that the value of the osmotic second virial coefficient tends to fall within a narrow range for 12 different proteins measured in dilute protein solutions containing components known to yield good crystals. Measurements made on the same proteins in the presence of ineffective precipitating agents yielded values of this thermodynamic variable well outside this 'crystallization' range. This technique may allow one to predict which conditions have the best chances of yielding suitable crystalline material.
The influence of solvent constituents on crystal growth were described by M. Frey (water in crystallization) and A. Ducruix (anions and cations). Other investigators examined the morphology of crystal growth and dissolution using high resolution microscopy (L. Monaco), laser Michelson interferometry (P. Vekilov), and atomic force microscopy, AFM (S. Durbin and K. Ward). P. Vekilov determined that at a critical value of supersaturation, the growth mechanism on the (101) face of hen eggwhite lysozyme changed from dislocation layer generation to 2D surface nucleation and that the slow growth of protein crystals is due to the impeded rate of molecules entering the growth site and not to low-solute (protein) concentration. S. Durbin measured step velocities, and observed that at low supersaturation, defect-mediated growth modes dominated two-dimensional nucleation mechanisms and inhibited growth at a particular defect by increasing the force applied to the surface by the AFM tip. Careful data processing and image analysis have allowed Ward and his co-workers to use AFM to obtain images of faces of lysozyme crystals, in situ, at molecular resolution.

Useful new precipitating agents described included mono-methylether PEG (S. Tolley), and a spectrum of 30 new detergents (B. Cudney). Strategies for efficient screening of crystallization conditions were described by A. D'Arcy (a Robot with a new plate), E. Stura (applied his 'reverse screening' system to a series of proteins), S. Sarfaty (sparse matrix and metallic screening sets), and C. Carter (multidimensional factorial approach with response surface methodologies and minimum predicted variance' in experimental design).

An additional session addressed resources vital to the crystal growth community including synchrotron radiation facilities (Å. Kvick and E. Westbrook), crystal databases (G. Gilliland), and new software for robotics (A. Hassel and A. Mistry). Proceedings of the San Diego Conference will appear in a special volume of refereed papers in Acta Crystallographica D.

K. Ward