The lung lamellar body as a functioning membrane in protein-catalyzed phosphatidylcholine transfer

Phosphatidylinositol transfer protein in lung: cellular and subcellular localization

Determination of the cellular distribution of phosphatidylinositol transfer protein in rat lung by immunocytochemistry revealed that the protein is more readily observed in the nonciliated bronchial epithelial cells (Clara cells) than in other lung cells. By light microscopy, the phosphatidylinositol transfer protein (PtdIns-TP) was localized to the dome-shaped apical region of Clara cells that were identified by staining with an antibody to Clara cell protein. Further investigation by electron microscopy revealed that the PtdIns-TP accumulated at the limiting membrane surrounding secretory granules and at the apical plasma membrane. This localization is compatible with the proposed roles for PtdIns-TP in formation of vesicles and exocytosis of secretory granules and, when considered in the context of the proposed role of PtdIns-TP in phosphatidylinositide metabolism, suggests that phosphatidylinositides may be involved in the mechanisms regulating Clara cell secretion.

Transfer of phospholipids by a protein fraction obtained from canine pulmonary lavage

Surfactant phospholipid exists in multicompartment pools within the subphase of the lung. Movement among these pools and back into type II alveolar cells may be catalyzed by a phospholipid transfer protein resident in the subphase. We demonstrate here that a protein fraction obtained from canine lung lavage catalyzes the intermembrane transfer of all the major surfactant phospholipids. The protein is probably not derived from serum and is unrelated to surfactant proteins that have already been described.

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.

Phospholipid transfer proteins from lung, properties and possible physiological functions

Phospholipid transfer proteins have been found in lung just as they have in tissues throughout the body. There is speculation that the proteins are involved in membrane biogenesis and in determining the phospholipid composition of membranes. For this reason the lung, which contains subcellular organelles of distinct phospholipid composition, is of interest in terms of its complement of phospholipid transfer proteins. The lamellar bodies of pulmonary type II alveolar cells have a phospholipid composition unique in terms of the proportions of dipalmitoyl phosphatidylcholine and phosphatidylglycerol. Studies of the phospholipid transfer proteins in lung have demonstrated two molecular species of the transfer proteins that differ significantly from those found in liver and other tissues. These proteins show specificity for the transfer of dipalmitoyl phosphatidylcholine and phosphatidylglycerol.

Protein composition of rabbit alveolar surfactant subfractions

The goal of this investigation was to characterize the proteins in subfractions of alveolar surfactant obtained by lung lavage and separated by differential centrifugation. It was previously demonstrated that the material in the more sedimentable fraction, which was enriched in tubular-myelin and was surface-active may be a precursor to the less sedimentable, vesicular, inactive material [1]. Separation of the proteins by polyacrylamide gel electrophoresis showed that the more sedimentable subfractions and rabbit surfactant isolated by conventional methods contained proteins with molecular weights comparable to those previously reported for alveolar surface active material (approximately 36 000 and 10 000). The less sedimentable subfractions contained less of these proteins. Immunoblots with anti-dog surfactant apoprotein antibodies, which cross-react with rabbit proteins, supported these observations. Immunoblots also showed that all of the subfractions contained serum proteins and secretory IgA, with the less sedimentable subfractions containing more secretory IgA. These results suggested that changes in protein composition may accompany functional changes in surfactant in the alveoli.

Acyl-chain specificity and membrane fluidity. Factors which influence the activity of a purified phospholipid-transfer protein from lung

A purified phospholipid-transfer protein from rat lung has been characterized in terms of the specificity of the protein for phosphatidylcholine molecules with different apolar moieties. The study demonstrated that the lung-phospholipid-transfer protein discriminates between dipalmitoylphosphatidylcholine and molecular species of phosphatidylcholine with unsaturated acyl chains. The initial rate of transfer of dipalmitoylphosphatidylcholine is 1.5-fold greater than the rate of transfer of dioleoylphosphatidylcholine, 1-palmitoyl-2- arachidonylphosphatidylcholine , or egg phosphatidylcholine under most assay conditions. Although the protein preferentially transfers dipalmitoylphosphatidylcholine, the incorporation of increasing mole percentages of dipalmitoylphosphatidylcholine into unilamellar phosphatidylcholine vesicles profoundly affects their effectiveness as donors for phosphatidylcholine transfer by the transfer protein. At 60 mol% dipalmitoylphosphatidylcholine, the rate of transfer is one-third that observed when vesicles are composed of 100% egg phosphatidylcholine. Decreases in membrane fluidity as estimated by fluorescence polarization of 1,6-diphenyl-1,3,5-hexatriene correlate with decreases in the effectiveness of the vesicles as donors in the phospholipid-transfer reaction. The conclusion from these studies is that the rate of transfer of phosphatidylcholine by the purified phospholipid-transfer protein from lung is determined by physical properties of membrane interfaces with which the protein interacts, as well as by the specificity of the phospholipid-transfer protein for different molecular species of phosphatidylcholine.

Properties of a non-specific phospholipid-transfer protein purified from rat lung

The present report describes the purification and characterization of a non-specific phospholipid-transfer protein from rat lung. The protein is the major phospholipid-transfer protein in lung which transfers phosphatidylcholine. The transfer protein was purified 1200-fold, with a final yield of 3%. The activity of the protein was monitored by measuring the transfer of [14C]phosphatidylcholine from radioactively labeled liposomes to mitochondria. The purified proteins transfers phosphatidylcholine, phosphatidylinositol, phosphatidylserine and phosphatidylethanolamine from radioactively labeled microsomes to either mitochondria or liposomes. The transfer of each phospholipid is proportional to its content in the donor membrane. The protein was purified from a pH 5.1 supernatant preparation by fractionation on DEAE-cellulose, Sephadex G-75 and hydroxyapatite. The molecular weight of the purified protein was estimated as 35 000 by SDS-polyacrylamide gel electrophoresis. The amino acid analysis revealed a high content of glutamic acid (including glutamine) and glycine. The specificity of the purified protein for transfer of phospholipids suggests that it may be the phospholipid-transfer activity which is highly enriched in isolated type II alveolar cells of rat lung.