Structure and function of collectins: humoral C-type lectins with collagenous regions.

Collectins are a family of C-type lectins with collagenous regions. Five such lectins have now been described: Lung surfactant protein A and D (SP-A and SP-D), and the plasma proteins, conglutinin, mannan-binding protein (MBP), and CL-43. They are composed of trimeric subunits containing a collagenous section and a C-terminal globular carbohydrate-recognizing domain containing the 14 invariant amino acids characteristic of the C-type lectins. The complete molecules of MBP and SP-A are composed of up to six such subunits, while conglutinin, SP-D and CL-43 contain up to four subunits. The collectins bind to carbohydrates on yeast, bacteria and viruses. The collagenous section reacts with the C1q receptor (collectin receptor) found on many cells including phagocytes. One function of these lectins appears to be the enhancement of phagocytosis, i.e. opsonization. One of the collectins, mannan-binding protein, activates the C1r2C1s2 complex of the classical complement pathway independently of C1q. Collectins have been found in several mammalian species and in the chicken. It seems likely that the biological role of the collectins is to participate in the innate immune defense.