Collectins: Mannan-Binding Protein as a Model Lectin

Abstract

A group of collagenous type of lectins: Collectins are a small family of secreted oligomeric glycoproteins that contain in common an NH2-terminal collagen-like domain and a COOH-terminal lectin (carbohydrate binding) domain (collagenous + lectin domains = collectin), and found in both lung and serum. The name collectin is derived from the words “collagen” and “lectin.” Collectins are C-type lectins that contain a collagen-like domain and usually assemble in large oligomeric complexes containing 9–27 subunits. They belong to C-type lectins family and have an important function in innate immunity, recognizing and binding to microorganisms via sugar arrays on the microbial surface. To date, nine different collectins have been identified: mannan-binding lectin (MBL), surfactant protein A (SP-A), surfactant protein D (SP-D), collectin liver 1 (CL-L1), collectin placenta 1 (CL-P1), conglutinin, collectin of 43 kDa (CL-43) and collectin of 46 kDa (CL-46), and collectin from kidney (CL-K1). The collectins MBP, conglutinin, CL-43, CL-46, CL-K1, SP-A, and SP-D are soluble, whereas CL-L1 and CL-P1 are membrane proteins. Some collectins, such as MBP and SP-A, organize into a “bouquet,” and others, such as bovine conglutinin and SP-D, organize into a “cruciform” shape. One of the best-studied serum collectins is MBP. Bovine CL-43 is structurally one of the simplest collectins, consisting of only three polypeptides, each of which contains a terminal CTLD. Rats have two serum MBPs designated A and C, sometimes called mannan-binding proteins. Humans appear to have only a single MBP corresponding to the rat MBP-A. The collectins can be classed into two distinct group, with MBP and SP-A being hexamers and SP-D, conglutinin and collectin-43 (CL-43) being tetramers, with proteins in the latter group also having significantly larger dimensions with respect to the length of their collagen-like ‘stalks’. The structural and functional relationships of this group of collectins have been reviewed (Hakansson and Reid 2000; Hansen and Holmskov 2002; and others).