Rotational dynamics of spin-labelled surfactant-associated proteins SP-B and SP-C in dipalmitoylphosphatidylcholine and dipalmitoylphosphatidylglycerol bilayers

Pulmonary surfactant proteins SP-B and SP-C have been isolated from porcine lungs and selectively labelled with 2,2,6,6-tetramethylpiperidine-N-oxyl ( TEMPO)-isothiocyanate at their N-terminal amine ends, to analyse the mobili ty of both proteins on the nanosecond time scale using electron spin resona nce (ESR) spectroscopy. Reconstitution of the labelled forms of these prote ins in bilayers of dipalmitoylphosphatidylcholine (DPPC) or dipalmitoylphos phatidylglycerol (DPPG) results in much broader and anisotropic ESR spectra , indicating a large restriction in rotational mobility of the protein-atta ched probe when inserted in membranes. Distinctive differences were found b etween the ESR spectra of the two polypeptides, that were consistent with i ntrinsic differences in mode of interaction of SP-B and SP-C with phospholi pid bilayers. The mobility of the protein spin probes was sensitive to temp erature on the time scale of conventional spin-label ESR. Both proteins, TE MPO-SP-B and TEMPO-SP-C, showed considerable increases in mobility at tempe ratures above the pretransition of purl DPPC. Finally, the mobility of the spin probes attached to both SP-B and SP-C was more restricted in DPPG than in DPPC bilayers, demonstrating that electrostatic interactions of the pos itively charged residues at the protein surface influence the rotational dy namics of the proteins in anionic lipid bilayers. Although some residual se gmental mobility of the thiourea-linked probes cannot be discounted, the re sults clearly reflect preferential differences in overall protein dynamics in gel and fluid phases of the two phospholipids that could be important fo r the biophysical properties of surfactant bilayers and monolayers. (C) 199 8 Elsevier Science B.V. All rights reserved.