The Structural Plasticity Of Lung Surfactant Peptide KL4 In Lipid Membranes

Surfactant protein B, SP-B, is critical to lung function and is particularly important in the trafficking of lipids within pulmonary surfactant and altering lipid properties at the air-water interface. The N- and C-terminal segments of SP-B have been identified as the most active domains in SP-B and remedial efforts in treating respiratory distress have focused on these domains and synthetic analogs of them. KL4 is a 21-residue peptide mimetic of the C-terminus of SP-B. The periodicity of the lysine residues in KL4 should prevent formation of a canonical amphipathic α-helix at lipid interfaces, yet upon partitioning into membranes CD measurements suggest formation of a helix. Using a suite of ssNMR experiments, in concert with circular dichroism spectroscopies, we are developing a molecular level understanding of the varied structure and function of KL4 and its parent sequence, SP-B59-80. In particular, our results highlight their lipid-dependent plasticity and unusual amphipathic helical secondary structures. We will present structural data obtained with solid-state NMR measurements which can resolve two helical conformations in KL4 with backbone torsion angles that deviate from a traditional α-helix and highlight the adaptive structure of amphipathic helices. We will also present phosphorous and deuterium NMR lineshape data which demonstrate the concentration dependent effects of SP-B related peptides on lipid dynamics in POPC/POPG and DPPC/POPG lipid lamellae. Our observations suggest a means for the peptides to penetrate deeply into lipid environments containing a high percentage of saturated lipids and to bind more peripherally to vesicles containing higher levels of unsaturated lipids. The adaptive structure and penetration depth of SP-B related peptides could explain the mechanism of action of SP-B and demonstrate the structural variability possible for amphipathic helices in lipid bilayer environments