Crystallization and preliminary X-ray diffraction analysis of a biologically active fragment of CD55

Crystals have been grown of two of the domains of CD55. This is the first r eport of crystallization of a short consensus repeat (SCR) domain containin g protein. CD55 is a widely expressed polymorphic glycoprotein, which funct ions as a complement regulator by inhibiting assembly and promoting destruc tion of C3 and C5 convertases. As a key regulator of complement, CD55 is im plicated in the hyperacute rejection of xenografts from pigs into primates. It is also commonly hijacked as a receptor by viruses (e.g. medically impo rtant echoviruses and coxsackieviruses) and bacterial pathogens (e.g. certa in pathogenic strains of Escherichia coli). Here, crystallization of a viru s-binding fragment expressed in yeast, consisting of two of the four extrac ellular SCR domains of CD55, is reported. The recombinant domains have been crystallized in 30% polyethylene glycol (PEG), 0.2 M sodium acetate, 0.1 M sodium acetate trihydrate pH 4.6 using the sitting-drop vapour-diffusion m ethod. Two crystal forms are observed (orthorhombic and monoclinic) and a n ative data set to 1.65 Angstrom resolution has been collected from the mono clinic form at the Synchrotron Radiation Source, Daresbury, UK.