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Intermolecular interaction of phosphatidylinositol with the lipid raft molecules sphingomyelin and cholesterol

View Article: PubMed Central - PubMed

ABSTRACT

Diacylphosphatidylinositol (PI) is the starting reactant in the process of phosphatidylinositide-related signal transduction mediated through the lipid raft domain. We investigated intermolecular interactions of PI with major raft components, sphingomyelin (SM) and cholesterol (Chol), using surface pressure–molecular area (π–A) isotherm measurements. The classical mean molecular area versus composition plot showed that the measured mean molecular areas are smaller in PI/Chol mixed monolayers and larger in PI/SM mixed monolayers than those calculated on the basis of the ideal additivity. These results indicate that PI interacts attractively with Chol and repulsively with SM. In addition, we energetically evaluated the interaction of PI with SM/Chol mixtures and found that the mixing energy of PI/SM/Chol ternary monolayers decreased as the molar ratio of Chol to SM increased. In order to quantitatively analyze the distribution of PI we calculated the chemical potentials of mixing of PI into the SM/Chol mixed monolayer and into the dioleoylphosphatidylcholine (DOPC) monolayer, which was used as a model for the fluid matrix, on the basis of partial molecular area analysis. Analysis using the chemical potential of mixing of PI suggested that partition of PI molecules between these two monolayers can be changed by a factor of about 1.7 in response to change in Chol molar fraction in the SM/Chol mixed monolayer from 0.3 to 0.6 when the concentration of PI in the DOPC monolayer is kept constant at 7 mol%.

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Intermolecular interaction in the PI/Chol monolayer system. (a) π–A isotherms of pure PI, pure Chol and PI/Chol mixed monolayers on the water subphase at 25±0.1°C. The molar fractions of PI, XPI, are indicated in the figure; 0 (Chol), 0.3, 0.5, 0.7, 0.9 and 1.0 (PI). (b) Mean molecular area versus composition analysis at 30 mN/m. The dotted line represents area additivity (Eq. (1)). (c) Areal compressional modulus () versus composition analysis. The  values at 30 mN/m were calculated from equation (4). The solid line represents ideal additivity of compressibility (see Materials and methods and Eq. (5)).
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f2-4_1: Intermolecular interaction in the PI/Chol monolayer system. (a) π–A isotherms of pure PI, pure Chol and PI/Chol mixed monolayers on the water subphase at 25±0.1°C. The molar fractions of PI, XPI, are indicated in the figure; 0 (Chol), 0.3, 0.5, 0.7, 0.9 and 1.0 (PI). (b) Mean molecular area versus composition analysis at 30 mN/m. The dotted line represents area additivity (Eq. (1)). (c) Areal compressional modulus () versus composition analysis. The values at 30 mN/m were calculated from equation (4). The solid line represents ideal additivity of compressibility (see Materials and methods and Eq. (5)).

Mentions: Secondly, we examined intermolecular interaction between PI and Chol, which is another essential component constituting the raft. The π–A isotherms for pure PI, pure Chol and PI/Chol mixed monolayers at 25±0.1°C are shown in Figure 2a. The pure Chol isotherm (leftmost in Fig. 2a) exhibited steep rise in the surface pressure at the molecular area of about 0.4 nm2/molecule, indicating that the gas phase is directly transformed into the LC phase28. In PI/Chol mixtures, the deviations from area additivity are always negative irrespective of XPI (Fig. 2b). Thus, in distinct contrast to SM, Chol induced the intermolecular condensation with PI. These results are consistent with the previous in vivo experiments that Chol depletion caused PI-dispersion from the Chol-rich domains (raft/caveola)21,22.


Intermolecular interaction of phosphatidylinositol with the lipid raft molecules sphingomyelin and cholesterol
Intermolecular interaction in the PI/Chol monolayer system. (a) π–A isotherms of pure PI, pure Chol and PI/Chol mixed monolayers on the water subphase at 25±0.1°C. The molar fractions of PI, XPI, are indicated in the figure; 0 (Chol), 0.3, 0.5, 0.7, 0.9 and 1.0 (PI). (b) Mean molecular area versus composition analysis at 30 mN/m. The dotted line represents area additivity (Eq. (1)). (c) Areal compressional modulus () versus composition analysis. The  values at 30 mN/m were calculated from equation (4). The solid line represents ideal additivity of compressibility (see Materials and methods and Eq. (5)).
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f2-4_1: Intermolecular interaction in the PI/Chol monolayer system. (a) π–A isotherms of pure PI, pure Chol and PI/Chol mixed monolayers on the water subphase at 25±0.1°C. The molar fractions of PI, XPI, are indicated in the figure; 0 (Chol), 0.3, 0.5, 0.7, 0.9 and 1.0 (PI). (b) Mean molecular area versus composition analysis at 30 mN/m. The dotted line represents area additivity (Eq. (1)). (c) Areal compressional modulus () versus composition analysis. The values at 30 mN/m were calculated from equation (4). The solid line represents ideal additivity of compressibility (see Materials and methods and Eq. (5)).
Mentions: Secondly, we examined intermolecular interaction between PI and Chol, which is another essential component constituting the raft. The π–A isotherms for pure PI, pure Chol and PI/Chol mixed monolayers at 25±0.1°C are shown in Figure 2a. The pure Chol isotherm (leftmost in Fig. 2a) exhibited steep rise in the surface pressure at the molecular area of about 0.4 nm2/molecule, indicating that the gas phase is directly transformed into the LC phase28. In PI/Chol mixtures, the deviations from area additivity are always negative irrespective of XPI (Fig. 2b). Thus, in distinct contrast to SM, Chol induced the intermolecular condensation with PI. These results are consistent with the previous in vivo experiments that Chol depletion caused PI-dispersion from the Chol-rich domains (raft/caveola)21,22.

View Article: PubMed Central - PubMed

ABSTRACT

Diacylphosphatidylinositol (PI) is the starting reactant in the process of phosphatidylinositide-related signal transduction mediated through the lipid raft domain. We investigated intermolecular interactions of PI with major raft components, sphingomyelin (SM) and cholesterol (Chol), using surface pressure–molecular area (π–A) isotherm measurements. The classical mean molecular area versus composition plot showed that the measured mean molecular areas are smaller in PI/Chol mixed monolayers and larger in PI/SM mixed monolayers than those calculated on the basis of the ideal additivity. These results indicate that PI interacts attractively with Chol and repulsively with SM. In addition, we energetically evaluated the interaction of PI with SM/Chol mixtures and found that the mixing energy of PI/SM/Chol ternary monolayers decreased as the molar ratio of Chol to SM increased. In order to quantitatively analyze the distribution of PI we calculated the chemical potentials of mixing of PI into the SM/Chol mixed monolayer and into the dioleoylphosphatidylcholine (DOPC) monolayer, which was used as a model for the fluid matrix, on the basis of partial molecular area analysis. Analysis using the chemical potential of mixing of PI suggested that partition of PI molecules between these two monolayers can be changed by a factor of about 1.7 in response to change in Chol molar fraction in the SM/Chol mixed monolayer from 0.3 to 0.6 when the concentration of PI in the DOPC monolayer is kept constant at 7 mol%.

No MeSH data available.