The ß Isoform of Human ATP-Binding Cassette B5 Transporter, ABCB5ß, Localizes to the Endoplasmic Reticulum.

ABCB5ß is a member of the ABC transporter superfamily cloned from melanocytes. It has been reported as a marker of skin progenitor cells and melanoma stem cells. ABCB5ß has also been shown to exert an oncogenic activity and promote cancer metastasis. However, this protein remains poorly characterized. To elucidate its subcellular localization, we tested several anti-ABCB5 antibodies and prepared several tagged ABCB5ß cDNA constructs. We then used a combination of immunofluorescence and biochemical analyses to investigate the presence of ABCB5ß in different subcellular compartments of HeLa and MelJuSo cell lines. Treatment of the cells with the proteasome inhibitor MG132 showed that part of the population of newly synthesized ABCB5ß is degraded by the proteasome system. Interestingly, treatment with SAHA, a molecule that promotes chaperone-assisted folding, largely increased the expression of ABCB5ß. Nevertheless, the overall protein distribution in the cells remained similar to that of control conditions; the protein extensively colocalized with the endoplasmic reticulum marker calnexin. Taken together with cell surface biotinylation studies demonstrating that the protein does not reach the plasma membrane (even after SAHA treatment), the data indicate that ABCB5ß is a microsomal protein predominantly localized to the ER.