Crystallization, Crystal Structure Analysis and Atomic Model of the Complex Formed by a Human Fc Fragment and Fragment B of Protein A fromStaphylococcus aureus

Crystals of the complex formed by human Fc fragment and fragment B (FB) of protein A from Staphylococcus aureus were prepared and the crystal structure determined at high resolution by multiple isomorphous replacement. Phase were improved considerably by combining these phases with calculated phases from the Fc component. FB is a small globular protein built of three parallel helices arranged in a triangular array. It binds by the first two helices of Fc and is attached to segments of CH2 and CH3. The CH3 module is unchanged between complex and Fc fragment crystals, but CH2 changes its position slightly relative to CH3. In addition, the upper third of CH2 is disordered in the complex crystals. Possible sources of this disorder are discussed.