Chimeras of surfactant proteins A and D identify the carbohydrate recognition domains as essential for phospholipid interaction.

Pulmonary surfactant proteins A (SP-A) and D (SP-D) possess similar structure as members of the mammalian C-type lectin superfamily. Both proteins are composed of four characteristic domains which are: 1) an NH2-terminal domain involved in interchain disulfide formation (denoted A1 domain for SP-A or D1 for SP-D); 2) a collagenous domain (denoted A2 or D2); 3) a neck domain (denoted A3 or D3); and 4) a carbohydrate recognition domain (denoted A4 or D4). SP-A specifically binds to dipalmitoylphosphatidylcholine, the major lipid component of surfactant, and can regulate the secretion and recycling of this lipid by alveolar type II cells. SP-D binds to phosphatidylinositol (PI) and glucosylceramide (GlcCer), and its role in alveolar lipid metabolism remains to be clarified. To understand the relationship between the structure and the function of both proteins with respect to their interaction with lipids, we expressed recombinant wild type rat SP-D (rSP-D) and chimeric molecules of SP-A and SP-D (A1A2A3D4, A1A2D3D4, and D1D2A3A4) using a baculovirus expression system, and performed lipid binding and aggregation assays. The rSP-D effectively competed with 125I-labeled native rat SP-D in a solid phase binding assay to PI and GlcCer in a manner nearly identical to native SP-D. The rSP-D also bound to PI liposomes with approximately half the affinity of native rat SP-D. Chimera A1A2D3D4 competed with iodinated SP-D in the solid phase binding assay to both PI and GlcCer. This chimera did not bind to dipalmitoylphosphatidylcholine (DPPC) liposomes or induce their aggregation. Chimera A1A2A3D4 did not bind solid phase PI or GlcCer but was equivalent to rSP-D in binding to PI liposomes. This chimera exhibited weak binding to DPPC but failed to aggregate DPPC liposomes. Chimera D1D2A3A4 failed to bind PI and GlcCer and bound weakly to DPPC liposomes but was quite effective at inducing aggregation of DPPC liposomes. These findings demonstrate that the D3 plus D4 domains of SP-D play a role in lipid binding and that the D4 domain is essential for PI binding. Furthermore, the A3 domain of SP-A cannot account for all the lipid binding activity of this protein. In addition, the results implicate the A4 domain of SP-A as an important structural domain in lipid aggregation phenomena.