Characterizing the Interaction of the ATP Binding Cassette Transporters (G subfamily) with the Intracellular Protein Lipid Environment

Cholesterol is an essential molecule that mediates a myriad of critical cellular processes, such as signal transduction in eukaryotes, membrane fluidity, and steroidogenesis. As such it is not surprising that cholesterol homeostasis is tightly regulated, striking a precise balance between endogenous synthesis and regulated uptake/efflux to and from extracellular acceptors. In mammalian cells, sterol efflux is a key component of the homeostatic equation and is mediated by members of the ATP binding cassette (ABC) transporter superfamily. ATP-binding cassette (ABC) transporters represent a group of evolutionarily highly conserved cellular transmembrane proteins that mediate the ATP-dependent translocation of substrates across membranes. Members of this superfamily, ABCA1 and ABCG1, are key components of the reverse cholesterol transport pathway. ABCG1 acts in concert with ABCA1 to maximize the removal of excess cholesterol from cells by promoting cholesterol efflux onto mature and nascent HDL particles, respectively. To date, mammalian ABC transporters are exclusively associated with efflux of cholesterol. In Saccharomyces cerevisiae, we have demonstrated that the opposite (i.e inward) transport of sterol in yeast is also dependent on two ABC transporters (Aus1p and Pdr11p). This prompts the question what dictates directionality of sterol transport by ABC transporters. The main focus of this study is to define the parameters that result in sterol movement across membranes. The comparison between these contrasting states (outward v. inward transport of the same substrate) will allow us to dissect whether sterol transport across the plasma membrane is defined by the molecule (i.e. the ABC transporter) or by microenvironment (i.e. the status of other proteins and lipids) in which it resides. We have developed the model eukaryote Saccharomyces cerevisiae as a tool to understand the mechanisms that influence ABC-transporter mediated movement of sterols. Specifically, we expressed murine ABCG1 (mABCG1) in yeast and assessed how changes in the intracellular sterol environment affect movement of sterols by this transporter. We found that expression of mABCG1 is able to vary (both increase and decrease) the concentration of exogenous sterols in the cell in response to intracellular sterol changes. We also found that yeast members of the ABCG subfamily, Aus1p and Pdr11p are able to promote either influx of cholesterol or efflux of a cholesterol derivative depending on the sterol context of the cell. This is the first example of an ABC transporter mediating bi-directional transport. These data suggest that direction of transport is not a static property of the transporter but rather can adapt in response to changes in the intracellular microenvironment. In addition to sterols we also found that proteins in the microenvironment may also influence direction of transport. Specifically, we found that the yeast sterol esterifying enzyme Are2p, physically interacts with the ABC transporters Aus1p and Pdr11p. Furthermore, all three proteins were found to co-localize to detergent resistant membrane microdomains. Deletion of either ABC transporter resulted in Are2p re-localization from DRMs to a detergent soluble fraction as well as a significant decrease in the percent of sterol esterified. This phenomenon is evolutionarily conserved in the murine lung where ABCG1 and ACAT1 were observed to co-localize with flotillin-1, a marker of DRMs. We propose that co-localization and complex formation of sterol esterification enzymes and ABC transporters in DRMs reflects a novel mechanism that directs membrane sterols to the esterification reaction. The studies presented in this thesis provide evidence that direction of transport is not a static inherent property of the transporter, but rather that it is mutable and influenced by surrounding sterols and proteins. The data provided here offers further insight as to how ABC transporters move cholesterol from the membrane and therefore may provide a platform for innovative strategies to combat atherosclerosis.