Crystallization and preliminary X‐ray diffraction studies of the carbohydrate‐recognition domain of SIGN‐R1, a receptor for microbial polysaccharides and sialylated antibody on splenic marginal zone macrophages

SIGN‐R1, or CD209b, is a mouse C‐type lectin receptor that is expressed at high levels on macrophages in lymphoid tissues, especially within the marginal zone of the spleen. SIGN‐R1 can bind and mediate the uptake of various microbial polysaccharides, including dextrans, lipopolysaccharides and pneumococcal capsular polysaccharides. It has been shown that SIGN‐R1 mediates the clearance of encapsulated pneumococcus, complement fixation via binding C1q independent of antibody and innate resistance to pneumococcal infection. Recently, SIGN‐R1 has also been demonstrated to bind sialylated antibody and mediate its activity to suppress autoimmunity. The carbohydrate‐recognition domain (CRD) of SIGN‐R1 has been cloned and overexpressed in a soluble secretory form in mammalian Chinese hamster ovary (CHO) cells. The CRD protein of SIGN‐R1 was purified from CHO cell‐culture supernatant and concentrated for crystallization using the hanging‐drop vapour‐diffusion method at 291 K. Crystals grew from a mixture of 2  M ammonium sulfate in 0.1  M bis‐tris pH 5.5. Single crystals, which belonged to the monoclinic space group C 2 with unit‐cell parameters a = 146.72, b = 92.77, c = 77.06 Å, β = 121.66°, allowed the collection of a full X‐ray data set to a maximum resolution of 1.87 Å.