ABCG2 Transports and Transfers Heme to Albumin through Its Large Extracellular Loop*

Abstract

ABCG2 is an ATP-binding cassette (ABC) transporter preferentially expressed by immature human hematopoietic progenitors. Due to its role in drug resistance, its expression has been correlated with a protection role against protoporhyrin IX (PPIX) accumulation in stem cells under hypoxic conditions. We show here that zinc mesoporphyrin, a validated fluorescent heme analog, is transported by ABCG2. We also show that the ABCG2 large extracellular loop ECL3 constitutes a porphyrinbinding domain, which strongly interacts with heme, hemin, PPIX, ZnPPIX, CoPPIX, and much less efficiently with pheophorbide a, but not with vitamin B12. K d values are in the range 0.5-3.5 M, with heme displaying the highest affinity. Nonporphyrin substrates of ABCG2, such as mitoxantrone, doxo/ daunorubicin, and riboflavin, do not bind to ECL3. Single-point mutations H583A and C603A inside ECL3 prevent the binding of hemin but hardly affect that of iron-free PPIX. The extracellular location of ECL3 downstream from the transport sites suggests that, after membrane translocation, hemin is transferred to ECL3, which is strategically positioned to release the bound porphyrin to extracellular partners. We show here that human serum albumin could be one of these possible partners as it removes hemin bound to ECL3 and interacts with ABCG2, with a K d of about 3 M.

A New ABC Half-Transporter in Leishmania major Is Involved in Resistance to Antimony

“…In addition, ABCI4 overexpression confers resistance to ZPP, a validated fluorescent heme analog transported by ABCG2 (46). Similarly, the resistance to antimonials [Sb(III)] was found to be due to a lower accumulation as a consequence of an increased ABCI4-mediated Sb(III) efflux activity; similar observations were obtained for Cd(II).…”

TMEM14C is required for erythroid mitochondrial heme metabolism

“…Our current data extend the findings that transport proteins play a critical role in heme homeostasis (52,(59)(60)(61)(62)(63). Although the National Human Genome Research Institute human genomewide association studies database (64) and genome-wide association studies have yet to link mutations in TMEM14C to hematologic disease (65)(66)(67), the profound anemic and mild porphyric phenotypes in Tmem14c gt/gt mice indicate that TMEM14C could function as a genetic modifier for the severity of anemia and porphyria in humans.…”

Shu1 Is a Cell-surface Protein Involved in Iron Acquisition from Heme in Schizosaccharomyces pombe

“…Shu1 Is Required for Assimilation of the Heme Analog ZnMP-To obtain further evidence that Shu1 supported heme assimilation, we investigated whether the presence of Shu1 led to cellular accumulation of fluorescent ZnMP, a heme analog (41)(42)(43). After removal of ALA to repress heme biosynthesis, hem1⌬ shu1⌬ mutant cells were preincubated for 3 h under iron-deficient or iron-replete conditions.…”