Collectin structure: A review

Collectins are animal calcium dependent lectins that target the carbohydrat e structures on invading pathogens, resulting in the agglutination and enha nced clearance of the microorganism. These proteins form trimers that may a ssemble into larger oligomers. Each polypeptide chain consists of four regi ons: a relatively short N-terminal region, a collagen like region, an alpha -helical coiled-coil, and the lectin domain. Only primary structure data ar e available for the N-terminal region, while the most important features of the collagen-like region can be derived from its homology with collagen. T he structures of the alpha-helical coiled-coil and the lectin domain are kn own from crystallographic studies of mannan binding protein (MBP) and lung surfactant protein D (SP-D). Carbohydrate binding has been structurally cha racterized in several complexes between MBP and carbohydrate; all indicate that the major interaction between carbohydrate and collectin is the bindin g of two adjacent carbohydrate hydroxyl group to a collectin calcium ion. I n addition, these hydroxyl groups hydrogen bond to some of the calcium amin o acid ligands. While each collectin trimer contains three such carbohydrat e binding sites, deviation from the overall threefold symmetry has been dem onstrated for SP-D, which may influence its binding properties. The protein surface between the three binding sites is positively charged in both MBP and SP-D.