Condensing Effect of Cholesterol on hBest1/POPC and hBest1/SM Langmuir Monolayers

Abstract

Human bestrophin-1 protein (hBest1) is a transmembrane channel associated with the calcium-dependent transport of chloride ions in the retinal pigment epithelium as well as with the transport of glutamate and GABA in nerve cells. Interactions between hBest1, sphingomyelins, phosphatidylcholines and cholesterol are crucial for hBest1 association with cell membrane domains and its biological functions. As cholesterol plays a key role in the formation of lipid rafts, motional ordering of lipids and modeling/remodeling of the lateral membrane structure, we examined the effect of different cholesterol concentrations on the surface tension of hBest1/POPC (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine) and hBest1/SM Langmuir monolayers in the presence/absence of Ca²⁺ ions using surface pressure measurements and Brewster angle microscopy studies. Here, we report that cholesterol: (1) has negligible condensing effect on pure hBest1 monolayers detected mainly in the presence of Ca²⁺ ions, and; (2) induces a condensing effect on composite hBest1/POPC and hBest1/SM monolayers. These results offer evidence for the significance of intermolecular protein-lipid interactions for the conformational dynamics of hBest1 and its biological functions as multimeric ion channel.

Keywords: Langmuir monolayers; POPC; cholesterol; condensing effect; hBest1; sphingomyelin.

Figure 1

Condensing effect of cholesterol on (A) hBest1, (B) hBest1/POPC, (C) hBest1/SM, (D) POPC and (E) SM monolayers (at molar fractions of Chol 0.167; 0.285; 0.375; 0.444 and 0.5) in the presence or absence of Ca²⁺, at 35 ± 2 °C. Data are presented as mean ± SE, n = 7.

Figure 2

BAM images of (A) hBest1/Chol monolayers, (B) hBest1/POPC/Chol, and (C) hBest1/SM/Chol monolayers (at molar fractions of Chol 0.167; 0.285; 0.375; 0.444 and 0.5) in the presence or absence of Ca²⁺, at 35 ± 2 °C. The white scale bar = 100 µm.