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Abstract
The ATP-binding cassette sub-family G member 1 (ABCG1) exports cellular cholesterol to high-density lipoproteins (HDL). However, a number of recent studies have suggested ABCG1 is predominantly localised to intracellular membranes. In this study, we found that ABCG1 was organized into two distinct cellular pools: one at the plasma membrane and the other associated with the endoplasmic reticulum (ER). The plasma membrane fraction was organized into filamentous structures that were associated with cortical actin filaments. Inhibition of actin polymerization resulted in complete disruption of ABCG1 filaments. Cholesterol loading of the cells increased the formation of the filamentous ABCG1, the proximity of filamentous ABCG1 to actin filaments and the diffusion rate of membrane associated ABCG1. Our findings suggest that the actin cytoskeleton plays a critical role in the plasma membrane localization of ABCG1.
Highlights
The ATP-binding cassette (ABC) sub-family G member 1 (ABCG1) is a transmembrane half transporter that exports cellular lipids to extracellular acceptors[1,2]
Whilst a number of investigators have reported that ABCG1 exports cholesterol to extracellular acceptors and that it is localized at the cell surface[4,12,13,14], others have reported that it functions intracellularly[16]
We utilized total internal reflection fluorescence (TIRF) microscopy and cell membrane sheet preparations to resolve these conflicting reports and establish unequivocally whether ABCG1 is localized at the plasma membrane and precisely which intracellular compartments with which it associates
Summary
The ATP-binding cassette (ABC) sub-family G member 1 (ABCG1) is a transmembrane half transporter that exports cellular lipids to extracellular acceptors[1,2]. A number of studies have reported that ABCG1 is localized and functions in lipid rafts in the plasma membrane[4,12,13,14,15] and in endosomes that recycle to the cell surface where they efflux cholesterol[13]. Given that ABCA1 and ABCG1 both efflux cholesterol from the cell surface to the extracellular space, and the requirement of ABCG1 dimerization for function, we hypothesized that ABCG1 can associate with actin. Support for this hypothesis comes from Aleidi et al.[20] who identified a number of cytoskeletal-associated proteins including alpha-tubulin, cytoskeleton-associated protein 5 and dynamin as ABCG1 interacting partners. We employed total internal reflection fluorescence microscopy together with plasma membrane sheets to establish that a sub-population of ABCG1 is localized to cortical actin filaments
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