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Protein patterns at lipid bilayer junctions.

ABSTRACT: We introduce a simple intermembrane junction system in which to explore pattern and structure formation by membrane-bound proteins. The junction consists of a planar lipid bilayer to which one species of protein (an IgG antibody) is bound, forming a 2D, compressible fluid. Upon the adhesion of a second lipid bilayer, the formerly uniformly distributed proteins rapidly reorganize into patterns of dense and sparse zones. Using a combination of complementary imaging techniques (fluorescence microscopy, fluorescence interference contrast microscopy, and fluorescence resonance energy transfer), we reconstruct the 3D structure of these intermembrane patterns with nanometer-scale topographic resolution, revealing the orientation of the proteins. The patterns form as the rapid bilayer-bilayer adhesion, often radiating outward from an initial, circular contact site, pushes aside the antibodies, sweeping them into areas of high density and clearing low-density regions. Coarsening of these local features is energetically costly and therefore kinetically trapped; the patterns do not change over tens of minutes. These studies demonstrate that membrane mechanical forces alone, i.e., in the absence of specific biochemical interactions, can drive microm-scale organization of membrane proteins.

SUBMITTER: Parthasarathy R

PROVIDER: S-EPMC516475 | biostudies-literature | 2004 Aug

REPOSITORIES: biostudies-literature

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Protein patterns at lipid bilayer junctions.

Parthasarathy Raghuveer R Groves Jay T JT

Proceedings of the National Academy of Sciences of the United States of America 20040820 35

We introduce a simple intermembrane junction system in which to explore pattern and structure formation by membrane-bound proteins. The junction consists of a planar lipid bilayer to which one species of protein (an IgG antibody) is bound, forming a 2D, compressible fluid. Upon the adhesion of a second lipid bilayer, the formerly uniformly distributed proteins rapidly reorganize into patterns of dense and sparse zones. Using a combination of complementary imaging techniques (fluorescence microsc ...[more]

PMID: 15322273

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Project description:Phosphatidylinositol-4,5-bisphosphate (PIP2), which constitutes ?1% of the plasma membrane phospholipid, plays a key role in membrane-delimited signaling. PIP2 regulates structurally and functionally diverse membrane proteins, including voltage- and ligand-gated ion channels, inwardly rectifying ion channels, transporters, and receptors. In some cases, the regulation is known to involve specific lipid-protein interactions, but the mechanisms by which PIP2 regulates many of its various targets remain to be fully elucidated. Because many PIP2 targets are membrane-spanning proteins, we explored whether the phosphoinositides might alter bilayer physical properties such as curvature and elasticity, which would alter the equilibrium between membrane protein conformational states-and thereby protein function. Taking advantage of the gramicidin A (gA) channels' sensitivity to changes in lipid bilayer properties, we used gA-based fluorescence quenching and single-channel assays to examine the effects of long-chain PIP2s (brain PIP2, which is predominantly 1-stearyl-2-arachidonyl-PIP2, and dioleoyl-PIP2) on bilayer properties. When premixed with dioleoyl-phosphocholine at 2 mol %, both long-chain PIP2s produced similar changes in gA channel function (bilayer properties); when applied through the aqueous solution, however, brain PIP2 was a more potent modifier than dioleoyl-PIP2. Given the widespread use of short-chain dioctanoyl-phosphoinositides, we also examined the effects of diC8-phosphoinositol (PI), PI(4,5)P2, PI(3,5)P2, PI(3,4)P2, and PI(3,4,5)P3. The diC8 phosphoinositides, except for PI(3,5)P2, altered bilayer properties with potencies that decreased with increasing head group charge. Nonphosphoinositide diC8 phospholipids generally were more potent bilayer modifiers than the polyphosphoinositides. These results show that physiological increases or decreases in plasma membrane PIP2 levels, as a result of activation of PI kinases or phosphatases, are likely to alter lipid bilayer properties, in addition to any other effects they may have. The results further show that exogenous PIP2, as well as structural analogues that differ in acyl chain length or phosphorylation state, alters lipid bilayer properties at the concentrations used in many cell physiological experiments.

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Protein patterns at lipid bilayer junctions.

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Protein patterns at lipid bilayer junctions.

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