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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/DOPC monolayer system. (a) π–A isotherms of pure PI, pure DOPC and PI/DOPC mixed monolayers on the water subphase at 25±0.1°C. The molar fractions of PI, XPI, are indicated in the figure; 0 (DOPC), 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)). The molecular area of DOPC (0.64 nm2) is larger than that of PI (0.55 nm2) at 30 mN/m.
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f3-4_1: Intermolecular interaction in the PI/DOPC monolayer system. (a) π–A isotherms of pure PI, pure DOPC and PI/DOPC mixed monolayers on the water subphase at 25±0.1°C. The molar fractions of PI, XPI, are indicated in the figure; 0 (DOPC), 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)). The molecular area of DOPC (0.64 nm2) is larger than that of PI (0.55 nm2) at 30 mN/m.

Mentions: Since the majority of lipids surrounding the rafts in bio-membranes are generally in the fluid state, comparative study between PI/raft component and PI/fluid lipid mixtures should be required. We used PI/DOPC monolayers because DOPC is one of the representative unsaturated phospholipids with the same headgroup as SM and the DOPC monolayer has been extensively used as a model for the fluid membrane11,39,40. The π–A isotherms for pure PI, pure DOPC and PI/DOPC mixed monolayers at 25±0.1°C are shown in Figure 3a. We evaluated the interaction between PI and DOPC molecules at 30 mN/m as described above. As a result, the deviation of Ames from area additivity was positive, indicating that the repulsive interaction was induced between PI and DOPC molecules (Fig. 3b). However, the deviations of Ames from area additivity in PI/DOPC monolayers were smaller than those in PI/SM monolayers. These results suggested that PI molecules have affinity in the order of Chol > DOPC > SM.


Intermolecular interaction of phosphatidylinositol with the lipid raft molecules sphingomyelin and cholesterol
Intermolecular interaction in the PI/DOPC monolayer system. (a) π–A isotherms of pure PI, pure DOPC and PI/DOPC mixed monolayers on the water subphase at 25±0.1°C. The molar fractions of PI, XPI, are indicated in the figure; 0 (DOPC), 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)). The molecular area of DOPC (0.64 nm2) is larger than that of PI (0.55 nm2) at 30 mN/m.
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Related In: Results  -  Collection

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f3-4_1: Intermolecular interaction in the PI/DOPC monolayer system. (a) π–A isotherms of pure PI, pure DOPC and PI/DOPC mixed monolayers on the water subphase at 25±0.1°C. The molar fractions of PI, XPI, are indicated in the figure; 0 (DOPC), 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)). The molecular area of DOPC (0.64 nm2) is larger than that of PI (0.55 nm2) at 30 mN/m.
Mentions: Since the majority of lipids surrounding the rafts in bio-membranes are generally in the fluid state, comparative study between PI/raft component and PI/fluid lipid mixtures should be required. We used PI/DOPC monolayers because DOPC is one of the representative unsaturated phospholipids with the same headgroup as SM and the DOPC monolayer has been extensively used as a model for the fluid membrane11,39,40. The π–A isotherms for pure PI, pure DOPC and PI/DOPC mixed monolayers at 25±0.1°C are shown in Figure 3a. We evaluated the interaction between PI and DOPC molecules at 30 mN/m as described above. As a result, the deviation of Ames from area additivity was positive, indicating that the repulsive interaction was induced between PI and DOPC molecules (Fig. 3b). However, the deviations of Ames from area additivity in PI/DOPC monolayers were smaller than those in PI/SM monolayers. These results suggested that PI molecules have affinity in the order of Chol > DOPC > SM.

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.