Abstract
Alveolar type II cells produce and secrete a complex mixture of lipids and proteins called pulmonary surfactant of which phospholipids are the major components. Surfactant proteins (SP) A, B, and C interact with phospholipids and are believed to play important roles in alveolar spaces. However, whether surfactant protein D (SP-D) interacts with phospholipids is unknown. In the present study, we examined whether SP-D binds to phospholipids and investigated phospholipid specificities of SP-D binding and the structural requirements of phospholipids for that binding using 125I-SP-D as a probe. 125I-SP-D bound exclusively to phosphatidylinositol (PI) in various phospholipids or a fraction containing phospholipids extracted from surfactant, which were developed on thin layer chromatography. 125I-SP-D also bound to PI coated on microtiter wells in a manner dependent upon the SP-D concentration. Unlabeled SP-D competed well with 125I-SP-D for PI binding and the antibody against SP-D abolished 125I-SP-D binding to PI. PI liposome also attenuated 125I-SP-D binding to the solid phase PI. Ca2+ is absolutely required for the binding of SP-D to PI. SP-D failed to bind to lyso-PI, fatty acids derived from PI digested with phospholipase A2, or diacylglycerol obtained after phospholipase C treatment of PI. SP-D bound to neither phosphatidylinositol 4-monophosphate nor phosphatidylinositol 4,5-diphosphate. We conclude that SP-D specifically binds to PI. This is the first report that demonstrates that SP-D interacts with surfactant phospholipids.
Highlights
Alveolar type I1 cells produce andsecrete a complex mixture of lipids and proteins called pulmonary surfactant of which phospholipids are the major components
We have recently demonstrated that Surfactant proteins (SP)-A specificallybinds to dipalmitoylphosphatidylcholine[16]
We have shown that three quarters of surfactant protein D (SP-D) in rat lung lavage exists in the supernatanot btained by centrifugation at 33,000 x g for 16 h [17], and that SP-Dcopurified with lipids from the supernatant counteracts the inhibitory effect of surfactant protein A (SP-A) on phosphatidylcholine secretion by alveolar type I1cells [18]
Summary
Alveolar type I1 cells produce andsecrete a complex mixture of lipids and proteins called pulmonary surfactant of which phospholipids are the major components. Surfactant proteins (SP) A, B, and C interact with phospholipids and are believed toplay important roles in alveolar spaces. Pulmonary surfactant is a complex mixture of phospholipids and proteins that is synthesized and secreted by alveolar type I1 cells [1]. SP-D is produced and secreted by alveolar type I1 cells [3, 4] and is regulated developmentally like the other surfactant components [11, 12]. This work provides direct evidence that SP-Dspecifically binds to phosphatidylinositol. This is the first report that demonstrates that SP-D interactswith surfactant phospholipids
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