DESIGNER LABELS; USEFUL TOOLS OR JUST ANOTHER FAD? Neil Q. McDonald⁺, Structural Biology Laboratory, Imperial Cancer Research Fund, Lincoln's Inn Fields, London WC2A 3PX; ⁺Department of Crystallography, Birkbeck College, P.O. Box 123, Malet Street, London WC1E 7HX, UK

At the present time the only crystallographic methods able to derive ab initio phase estimates for medium to large sized proteins require the incorporation of a few electron dense or alternatively, anomalous scattering centres into a protein. Such atoms must modify the observed structure factors to the extent that their positions can be identified allowing phase estimates of the protein to be obtained.

Various techniques have been described to introduce labels into a protein or protein complex, examples include (1) direct chemical modification (2) protein engineering and (3) in vivo labelling of recombinant proteins. In some cases the incorporation of such labels has been developed to solve a specific structural problem. Other approaches are emerging as being more generally applicable.

I will review current methods for protein labelling with an emphasis on multi-wavelength anomalous scattering. I shall focus on chemical modification and incorporation of selenomethionine referring to specific examples in my laboratory.