SURFACTANT PROTEIN-B - EFFECTS ON LIPID DOMAIN FORMATION AND INTERMEMBRANE LIPID FLOW

Pulmonary surfactant is a mixture of (phospho)lipids and surfactant sp ecific proteins, lining the alveolar space. During each respiration cy cle phospholipids are transferred between the phospholipid monolayer a t the air/water interface and a variety of underlying membranes. Surfa ctant proteins may play a role in facilitating the insertion and remov al of phospholipids by affecting the lipid organization of the bilayer and monolayer. The experiments described in this article were carried out in order to investigate the influence of surfactant protein B (SP -B) on the distribution of phospholipids in membranes and on the mixin g of lipids between membranes. To determine the distribution of the no n-labeled phospholipids in small unilamellar vesicles (SUV), the relat ive clustering of pyrene-labeled phospholipids was used, by measuring the ratio of excimer-to-monomer (E/M) pyrene fluorescence. In the abse nce of SP-B it was found that the clustering of the pyrenePC molecules was dependent on the proportion of saturated acyl chains and not on t he proportion of negative charges. Addition of the positively charged SP-B to a mixture of DPPC and PG, led to an increase of approximately 20% in E/M ratio, indicating a clustering of the negatively charged PG molecules. This effect was intensified by addition of calcium ions. I f pyrenePC-containing SUV were mixed with excess non-labeled SW in the presence of SP-B and calcium ions, the E/M ratio decreased, correspon ding with a flow of the pyrenePC molecules into the acceptor membranes . It is concluded that presence of domains of phospholipids can be det ected with the use of pyrene-labeled phospholipids. Furthermore, SP-B showed a concentrating effect on the distribution of the negatively ch arged phospholipids, a process that could be important in regulating t he phospholipid composition of the monolayer.