Lung surfactant protein D (SP-D) and the molecular diverted descendants: conglutinin, CL-43 and CL-46

Lung surfactant protein D (SP-D), conglutinin, CL-43 and CL-46 belong to a group of proteins designated collectins that, besides a common structure made of a collagen-like region and a C-type lectin domain, are important components of the innate immune defence. They all bind complex glycoconjugates on microorganisms thereby inhibiting infection, enhancing the clearance by phagocytes and modulating the immune response. In addition, SP-D inhibits the generation of radical oxygen species or the propagation of lipid peroxidation. Knock-out mice deficient in SP-D have a disturbed homeostasis of pulmonary surfactant and suffer from oxidative stress leading to pulmonary inflammation upon microbial challenge. Conglutinin, CL-43 and CL-46 have in contrast to the rest of the collectin family only been found in cattle. During the characterization of the genes encoding conglutinin, CL-43 and CL-46 we observed several features showing that the additional bovine collectins are diverted molecular descendants of an ancestral SP-D gene. Since structural similarity often associates with common functionality, some of SP-D's effector mechanisms may apply to conglutinin, CL-43 and CL-46--and vice versa. This review focus on the structural and functional relationship of this group of collectins.