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Journal of Lipid Research, Vol. 49, 1420-1430, July 2008
Copyright © 2008 by American Society for Biochemistry and Molecular Biology
Chemistry, Materials, and Life Sciences, Lawrence Livermore National Laboratory, Livermore, CA 94551
This work was performed under the auspices of the U. S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344 with support from Lawrence Livermore National Laboratory (Grant 06-SI-003 awarded to P.D.H.).
Published, JLR Papers in Press, April 9, 2008.
1 To whom correspondence should be addressed. e-mail: eadsulchek1{at}llnl.gov
Self-assembly of purified apolipoproteins and phospholipids results in the formation of nanometer-sized lipoprotein complexes, referred to as nanolipoprotein particles (NLPs). These bilayer constructs are fully soluble in aqueous environments and hold great promise as a model system to aid in solubilizing membrane proteins. Size variability in the self-assembly process has been recognized for some time, yet limited studies have been conducted to examine this phenomenon. Understanding the source of this heterogeneity may lead to methods to mitigate heterogeneity or to control NLP size, which may be important for tailoring NLPs for specific membrane proteins. Here, we have used atomic force microscopy, ion mobility spectrometry, and transmission electron microscopy to quantify NLP size distributions on the single-particle scale, specifically focusing on assemblies with 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and a recombinant apolipoprotein E variant containing the N-terminal 22 kDa fragment (E422k). Four discrete sizes of E422k/DMPC NLPs were identified by all three techniques, with diameters centered at
14.5, 19, 23.5, and 28 nm. Computer simulations suggest that these sizes are related to the structure and number of E422k lipoproteins surrounding the NLPs and particles with an odd number of lipoproteins are consistent with the double-belt model, in which at least one lipoprotein adopts a hairpin structure.
Supplementary key words apolipoproteins nanodiscs high density lipoproteins atomic force microscopy ion mobility spectrometry
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