The role of membrane remodeling in surfactant protein B (SP-B) function

Surfactant protein B (SP-B) is a hydrophobic, 79 amino acid peptide that regulates the structure and function of surfactant phospholipid membranes in the airspaces of the lung. Addition of SP-B to liposomes composed of DPPC/PG (7:3) leads to membrane binding, destabilization and fusion ultimately resulting in rearrangement of membrane structure. The hypothesis underlying this work is that the fusogenic and/or lytic properties of the 79 residue mature SP-B peptide are critical for (1) SP-C maturation, (2) organization of surfactant phospholipids in the distal secretory pathway, (3) formation and maintenance of a surface film and (4) maintenance of a sterile gas exchange surface. As a first step in determining the functional importance of SP-B mediated fusion and lysis, the fusogenic and lytic domains of SP-B were mapped. Synthetic peptides were generated to the predicted helices of SP-B and tested for liposome fusion and lysis. The molecular basis of these properties was determined by systematically introducing amino acid substitutions into the SP-B sequence to identify specific residues important for membrane fusion and lysis. Finally, the importance of these properties for SP-B function was assessed by analyzing the affect of altered fusion and lysis on the surface tension reducing properties of surfactant (Chapter II) and determining the importance of the lytic property of SP-B in its ability to kill airway pathogens (Chapter III).