Lung surfactant is a mixture of phospholipids, neutral lipids, and surfactant protein A (SP-A) 1 SP-B, SP-C, and SP-D, which are secreted into the air spaces by alveolar type II cells and Clara cells of the distal pulmonary epithelium (1). Although the primary function of surfactant is to reduce surface tension, the contribution of each molecular component to surface activity is not completely understood. Surfactant phospholipids form a film at the air-liquid interface that maintains air space patency by resisting compression as the alveolar radius decreases during expiration. Data from in vitro experiments, gene-targeted animals, and naturally occurring mutations in humans indicate that the hydrophobic surfactant proteins, SP-B and SP-C, participate in the assembly and biophysical properties of the surfactant film (2). The hydrophilic surfactant proteins, SP-A and SP-D, have a complex functional profile. The recognition that SP-A and SP-D are structurally homologous to mannosebinding protein has identified them as members of the collectin family of innate opsonins and directed attention to their host defense properties (3). Like mannose-binding protein, SP-A and SP-D bind to a wide range of microorganisms and enhance microbial phagocytosis and killing by alveolar macrophages. These in vitro activities appear to be physiologically relevant, since gene-targeted SP-A Ϫ/Ϫ and SP-D Ϫ/Ϫ mice clear microbial infections less effectively than pulmonary collectin-sufficient mice (4 -7). However, SP-A Ϫ/Ϫ and SP-D Ϫ/Ϫ mice also exhibit abnormalities of surfactant structure, metabolism and function (8 -10). Surfactant isolated from SP-A Ϫ/Ϫ mice does not contain the large aggregate tubular myelin and has impaired surface activity in the presence of plasma inhibitors (11). SP-D Ϫ/Ϫ mice develop progressive alveolar phospholipidosis and air space dilation (9, 10), associated with increased macrophage production of metalloproteinases and oxidant species (12). All of these defects are corrected by lung-specific expression of the cognate collectin in the SP-A Ϫ/Ϫ and SP-D Ϫ/Ϫ mice (13, 14). The structural basis of SP-A and SP-D surfactant functions has been explored by mutagenesis using in vitro and in vivo analyses. The primary structure of both proteins includes an N-terminal segment containing interchain linkages formed by Cys residues, a collagen-like region of Gly-X-Y repeats, a hydrophobic "neck" domain, and a carbohydrate recognition domain (CRD) (15,16). Trimeric association of subunits occurs by the folding of the collagen-like domains into triple helices (17) and coiled-coil bundling of ␣-helices in the neck (18). In the fully assembled molecules, the N-terminal sequences and di- ϩ/ϩ mice disrupted oligomeric assembly of the endogenous SP-D and produced air space dilation and foamy macrophage formation without phospholipidosis (24). These data suggested that the in vivo activity of SP-D is dependent on its oligomeric structure. The purpose of this study was to examine the role of the N-terminal segment-dependent olig...