ABCB5 Full Name
ATP-binding cassette, sub-family B (MDR/TAP), member 5
ABCB5 Introduction
ABCB5, also known as P-glycoprotein ABCB5 or ATP-binding cassette subfamily B member 5, is a member of the ABC transporter superfamily that was originally identified by homology to ABCB1 (MDR1/P-glycoprotein). The human ABCB5 gene is located on chromosome 7p21.1 within a cluster of ABCB family genes, including ABCB1 and ABCB4, suggesting that ABCB5 arose through gene duplication events. Alternative splicing generates multiple isoforms, the most well-characterized being a full-length transporter (ABCB5伪) and a shorter, N-terminally truncated isoform (ABCB5尾) that lacks the first transmembrane domain. The full-length protein is predicted to contain 1,257 amino acids with a molecular weight of approximately 138 kDa. Unlike the well-defined physiological functions of ABCB4 in biliary phosphatidylcholine transport, the endogenous substrate and transport direction of ABCB5 remain incompletely understood, though accumulating evidence suggests it participates in the efflux of specific lipid species, chemotherapeutic agents, and possibly regulators of cell differentiation.
Figure 1.The structure of ABCB5.
Tissue Distribution and Cellular Localization
ABCB5 expression is highly restricted and cell-type specific, with the most robust expression detected in a subset of tissue-resident stem cells and certain malignant tumors. The protein is prominently expressed in the limbus of the cornea, where it serves as a marker for limbal stem cells (LSCs) that maintain the corneal epithelium. Additionally, ABCB5 has been identified in melanocyte stem cells of the hair follicle, mesenchymal stem cells from adipose tissue and bone marrow, and subpopulations of dermal fibroblasts. At the subcellular level, ABCB5 localizes primarily to the plasma membrane, consistent with a role in transmembrane transport, but it also appears within CD34-positive intracellular compartments in melanocytic cells, suggesting involvement in vesicular trafficking and endosomal recycling. Promoter analysis reveals binding sites for transcription factors such as MITF (microphthalmia-associated transcription factor) in melanocytic lineages and PAX6 in corneal cells, explaining the restricted expression patterns. In the mouse, Abcb5 is also expressed in the testis, brain endothelium, and a subset of hematopoietic stem cells, though species differences exist; for example, murine Abcb5 does not directly substitute for ABCB5 functional studies in human melanoma due to sequence divergence in the drug-binding pocket.
Role in Stem Cell Maintenance and Corneal Homeostasis
One of the best-established physiological functions of ABCB5 is the maintenance of limbal stem cells essential for corneal epithelial renewal. ABCB5-positive (ABCB5+) limbal cells, identified by flow cytometry and immunohistochemistry, possess the hallmarks of adult stem cells: clonogenicity, self‑renewal, and the ability to generate fully stratified corneal epithelium upon transplantation. When ABCB5+ cells are isolated from human limbal tissue and transplanted into a mouse model of corneal injury (limbal stem cell deficiency, LSCD), they restore a clear, avascular cornea with donor-derived epithelial cells, whereas ABCB5‑negative cells fail to do so. Genetic ablation of Abcb5 in mice results in reduced limbal stem cell numbers, impaired corneal wound healing, and progressive corneal opacification, recapitulating features of human LSCD. These findings have led to the development of a cell‑therapy product (ABCB5+ limbal stem cells) that is currently undergoing clinical trials for patients with LSCD caused by chemical burns or ocular surface disorders. Beyond the cornea, ABCB5 marks melanocyte stem cells in the hair follicle bulge; in Abcb5‑knockout mice, the melanocyte stem cell pool is diminished, leading to premature graying of the fur and defective hair follicle pigmentation cycling.
Alternate Names for ABCB5
ABCB5
ATP-binding cassette, sub-family B (MDR/TAP), member 5
ABCB5beta
EST422562
ABCB5alpha
ATP-binding cassette sub-family B member 5
ABCB5 P-gp
P-glycoprotein ABCB5
ATP-binding cassette protein
