Endonexin (annexin IV)-mediated lateral segregation of phosphatidylglycerol in phosphatidylglycerol/phosphatidylcholine membranes.
Endonexin (annexin IV) is a member of the annexin family of homologous proteins that share the ability to bind to pure lipid membranes and to aggregate vesicles in a Ca(2+)-dependent fashion. Endonexin appears to preferentially interact with certain types of lipids such as phosphatidylglycerol (PG) in PG/phosphatidylcholine (PC) mixed lipid membranes. Such preferential binding should result in localization of PG lipids to membrane regions where endonexin is bound. This was tested using a PG derivative containing the fluorophore pyrene, which exhibits fluorescence sensitive to molecular collision frequency. Motional restriction of pyrene-PG upon endonexin-membrane binding was evident from decreased ratios of excimer-to-monomer (E/M) pyrene fluorescence with endonexin binding to 97% PG/3% pyrene-PG vesicles. A maximum decrease of 30% suggests a 30% decrease in the average diffusion constant of pyrene-PG molecules or a 53% decrease assuming that only outer-monolayer lipid molecules interact with endonexin. In vesicles containing 5% and 10% pyrene-PG in PC, segregation of lipids was evident from observed increases in E/M of 14.2 +/- 1.8% and 6.8 +/- 0.1%, respectively, in the presence of endonexin and either 10 mM (5% pyrene-PG) or 2 mM (10% pyrene-PG) free Ca2+. At higher concentrations of Ca2+ (> 10 mM for 5% pyrene-PG and > 2 mM for 10% pyrene-PG), smaller endonexin-dependent increases in E/M are observed as endonexin molecules at high surface densities compete for the limited pool of pyrene-PG. The nature of these interactions of endonexin with mixed lipid systems has implications for the way annexins may modulate membrane structure in cells.
Program in Biophysics, University of Virginia, Charlottesville 22908.
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Junker
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Creutz
D000818:Animals; D017314:Annexin A4 / Q000737:chemistry / Q000378:metabolism*; D002118:Calcium / Q000378:metabolism / Q000494:pharmacology; D002417:Cattle; D007700:Kinetics; D008081:Liposomes / Q000378:metabolism*; D008099:Liver / Q000378:metabolism*; D010713:Phosphatidylcholines / Q000378:metabolism*; D010715:Phosphatidylglycerols / Q000378:metabolism*; D011485:Protein Binding; D011721:Pyrenes; D013050:Spectrometry, Fluorescence / Q000379:methods
D017314:Annexin A4; D008081:Liposomes; D010713:Phosphatidylcholines; D010715:Phosphatidylglycerols; D011721:Pyrenes; D002118:Calcium