Promotion of Coalescence in Bicellar Mixtures by an SP-B Fragment

Abstract

Maintenance of a functional surfactant layer requires the transfer of material between bilayer reservoirs and the surface-active layer. It is believed that SP-B facilitates the interlayer contact and mixing implicit in such activity. SP-B's role in promoting these contacts was investigated though the use of bilayered micelle mixtures containing short- and long-chain lipids. Upon warming, these bicellar mixtures progressively coalesce into more extended structures and are thus an interesting system in which to study the capacity of polypeptides to promote interactions between lipid structures. 2H NMR was used to identify the capacity of perturbation by an SP-B fragment (SP-B63-78) upon bicellar mixtures of DMPC-d54/DMPG/DHPC and DMPC-d54/DMPC/DHPC in nominal 3:1:1 molar ratios. In the presence of mixtures containing anionic lipids (DMPC-d54/DMPG/DHPC), SP-B63-78 (concentration approximately 10% of lipid weight) was found to lower the temperature at which coalescence to extended lamellar structures occurred. Conversely, when anionic lipids were replaced with zwitterionic lipids (DMPC-d54/DMPC/DHPC), SP-B63-78 did not perturb the temperature at which the transition to extended lamellar structures occurred. These results indicate that the interaction of SP-B63-78 with model membranes is dependent upon the presence of anionic lipids and further suggest a mechanism by which full length SP-B may interact with membranes.