Assembly and Interfacial Behavior of Recombinant Surfactant Protein-B in Detergent Micelles and Lipid Monolayers.

Abstract

Although surfactant protein-B (SP-B) is essential for pulmonary surfactant function, the experimental structure of mature SP-B remains undetermined, and the molecular basis of its function is unclear. Here, we characterize the structural and functional behavior of a recombinant SP-B construct (rSP-B) in DPC/SDS detergent micelles and DPPC/POPG lipid monolayers. Light scattering showed that dilute DPC/SDS (9:1, 0.2% wt) is intrinsically heterogeneous, containing both micellar (~4 nm) and supramicellar structures (~68-400 nm), consistent with non-ideal surfactant mixing near the critical micelle concentration. Dialysis consolidated these populations into a single, reproducible mesoscale structure (~600 nm) that retained predominantly -helical structure, whereas disrupting these assemblies compromised protein folding, indicating a role for supramolecular organization in stabilizing rSP-B, although the resulting complex sizes exceed the regime typically accessible to solution NMR. Higher detergent concentrations instead favored a homogeneous micellar population (~4 nm), demonstrating that detergent concentration strongly influences protein-detergent complex (PDC) size. In parallel, rSP-B incorporated into DPPC/POPG monolayers markedly improved cyclic reversibility relative to protein-free films, converging to a stable, low-hysteresis profile with compressibility signatures consistent with reversible, elastic collapse and respreading through mechanical reorganization and out-of-plane deformation. These results inform strategies for optimizing detergent conditions for future structural studies and support a model in which rSP-B stabilizes mechanically coupled reservoirs essential for surfactant recycling.