Cellular localization and trafficking of the human ABCA1 transporter

ABCA1, the ATP-binding cassette protein,mutated in Tangier disease, mediate s the efflux of,excess cellular sterol to apoA-I and thereby the formation of high density lipoprotein. The intracellular localization and trafficking of ABCA1 was examined in stably and transiently transfected HeLa cells exp ressing a functional human ABCA1-green fluorescent protein (GFP) fusion pro tein. The fluorescent chimeric ABCA1 transporter was found to reside on the cell surface and on intracellular vesicles that include a novel subset of early endosomes, as well as late endosomes and lysosomes, Studies of the lo calization and trafficking of ABCA1-GFP in the presence of brefeldin A or m onensin, agents known to block intracellular vesicular trafficking as well as apoA-I-mediated cellular lipid efflux, showed that: (i) ABCA1 functions in lipid efflux at the cell surface, and (ii) delivery of ABCA1 to lysosome s for degradation may serve as a mechanism to modulate its surface expressi on. Time-lapse fluorescence microscopy revealed that ABCA1-GFP-containing e arly endosomes undergo fusion, fission,: and tubulation and transiently int eract with one another, late endocytic vesicles, and the cell surface, Thes e studies establish a complex intracellular trafficking pathway for human A BCA1 that may play important roles in modulating ABCA1 transporter activity and cellular cholesterol homeostasis.