Calcium ions as "miscibility switch": colocalization of surfactant protein B with anionic lipids under absolute calcium free conditions.
One of the main determinants of lung surfactant function is the complex interplay between its protein and lipid components. The lipid specificity of surfactant protein B (SP-B), however, and the protein's ability to selectively squeeze out lipids, has remained contradictory. In this work we present, for the first time to our knowledge, by means of time-of-flight secondary ion mass spectrometry chemical imaging, a direct evidence for colocalization of SP-B as well as its model peptide KL(4) with negatively charged dipalmitoylphosphatidylglycerol under absolute calcium free conditions. Our results prove that protein/lipid localization depends on the miscibility of all surfactant components, which itself is influenced by subphase ionic conditions. In contrast to our earlier studies reporting SP-B/KL(4) colocalization with zwitterionic dipalmitoylphosphatidylcholine, in the presence of even the smallest traces of calcium, we finally evidence an apparent reversal of protein/lipid mixing behavior upon calcium removal with ethylene diamine tetraacetic acid. In addition, scanning force microscopy measurements reveal that by depleting the subphase from calcium ions the protrusion formation ability of SP-B or KL(4) is not hampered. However, in the case of KL(4), distinct differences in protrusion morphology and height are visible. Our results support the idea that calcium ions act as a "miscibility switch" in surfactant model systems and probably are one of the major factors steering lipid/protein mixing behavior as well as influencing the protein's protrusion formation ability.
Saleem
Mohammed
M
Institute of Biochemistry, University of Münster, Münster, Germany.
Meyer
Michaela C
MC
Breitenstein
Daniel
D
Galla
Hans-Joachim
HJ
D000595:Amino Acid Sequence; D000818:Animals; D002118:Calcium / Q000737:chemistry / Q000378:metabolism*; D002462:Cell Membrane / Q000737:chemistry / Q000378:metabolism; D002614:Chelating Agents / Q000737:chemistry; D004492:Edetic Acid / Q000737:chemistry; D036341:Intercellular Signaling Peptides and Proteins; D050356:Lipid Metabolism; D008055:Lipids / Q000737:chemistry*; D013058:Mass Spectrometry; D018625:Microscopy, Atomic Force; D008969:Molecular Sequence Data; D010455:Peptides / Q000737:chemistry / Q000378:metabolism; D021381:Protein Transport; D037701:Pulmonary Surfactant-Associated Protein B / Q000737:chemistry / Q000378:metabolism*; D013552:Swine
D002614:Chelating Agents; D036341:Intercellular Signaling Peptides and Proteins; C097558:KL4 surfactant; D008055:Lipids; D010455:Peptides; D037701:Pulmonary Surfactant-Associated Protein B; D004492:Edetic Acid; D002118:Calcium