Phospholipid-protein interactions in rat lung lamellar bodies

We investigated the specificity of the cytosol-mediated phosphatidylcholine transfer between isolated rat lung microsomes and rat lung lamellar bodies. For that purpose we labeled the microsomes with 1-acyl-2-[1-14C]palmitoyl- and 1-acyl-2-[9,10-3H]oleoylphosphatidylcholine through protein-catalyzed phosphatidylcholine exchange. Incubation in buffer resulted in 3-5% transfer of label from microsomes to lamellar bodies. Lung cytosol stimulated this transfer about 2-fold and the presence of 12 micrograms/ml phosphatidylcholine-transfer protein from bovine liver resulted in a 30 to 35% recovery of radioactivity in the lamellar bodies. When microsomal donor membranes with a 3H/14C ratio of 2.6 were used, the 3H/14C ratios of the lamellar bodies were 3.9, 3.7 and 3.7, after incubation in buffer, with cytosol and with bovine liver exchange protein, respectively. Doubling the amount of lamellar body acceptor membranes resulted in 3H/14C ratios in the lamellar bodies of 4.6 and 4.1, after incubation in buffer and with cytosol, respectively. Furthermore, we isolated the protein component from rat lung lamellar bodies and performed reconstitution experiments with phospholipids. Reconstituted and non-reconstituted phospholipid and protein were separated by either Sepharose 4B gel filtration or discontinuous sucrose gradient centrifugation. The presence of lamellar body protein in the reconstitution mixture resulted in the formation of larger structures with higher density than those formed in control experiments without protein. When 1-acyl-2-[1-14C]palmitoyl- and 1-acyl-2-[9,10-3H]oleoylphosphatidylcholine were included in the reconstitution mixture, the structures containing lamellar body protein had 2- to 4-fold lower 3H/14C ratios than initially present in the incubation. These results suggest that lamellar body proteins associate preferentially with disaturated phosphatidylcholine species.