STRUCTURAL ASPECTS OF COLLECTINS AND RECEPTORS FOR COLLECTINS

The collectins are oligomeric molecules composed of C-type lectin doma ins attached to collagen regions via alpha-coiled neck regions. Five m embers of the collectins have been characterized. Mannan-binding lecti n (MBL), conglutinin and collectin-43 (CL-43) are serum proteins produ ced by the liver. Lung surfactant protein A (SP-A) and lung surfactant protein D (SP-D) are mainly found in the lung, where they are synthes ized by alveolar type II cells and secreted to the alveolar surface. T he collectins are believed to play an important role in innate immunit y. They bind oligosaccharides on the surface of a variety of microbial pathogens. After binding of the collectins to the microbial surface e ffector mechanisms such as agglutination, neutralizing or opsonization of the microorganisms for phagocytosis are initiated. SP-A and SP-D s timulate chemotaxis of phagocytes and once bound to the phagocytes, th e production of oxygen radicals can be induced. In the case of MBL the opsonization can be further enhanced by complement activation via the MBLectin pathway while conglutinin interacts with the complement syst em by binding to the complement degradation product iC3b. A number of receptors and binding molecules interacting with the collectins are fo und on the membrane or in association with the membrane of various cel ls responsible for phagocytosis and clearance of microorganisms. This paper focus on the structural aspects of the collectins and the recept ors for collectins.