The ABCA subfamily of ATP-binding cassette (ABC) transporters includes eleven members to date. In this study, we describe a new, unusually large gene on chromosome 7p12.3, ABCA13. This gene spans over 450 kb and is split into 62 exons. The predicted ABCA13 protein consists of 5,058 ami- no acid residues making it the largest ABC protein described to date. Like the other ABCA subfamily members, ABCA13 contains a hydrophobic, predicted transmembrane segment at the N-terminus, followed by a large hydrophilic region. In the case of ABCA13, the hydrophilic region is unexpectedly large, more than 3,500 amino acids, encoded by 30 exons, two of which are 4.8 and 1.7 kb in length. These two large exons are adjacent to each other and are conserved in the mouse Abca13 gene. Tissue profiling of the major transcript reveals the highest expression in human trachea, testis, and bone marrow. The expression of the gene was also determined in 60 tumor cell lines and the highest expression was detected in the SR leukemia, SNB-19 CNS tumor and DU-145 prostate tumor cell lines. ABCA13 has high similarity with other ABCA subfamily genes which are associated with human inherited diseases: ABCA1 with the cholesterol transport disorders Tangier disease and familial hypoalphalipoproteinemia, and ABCA4 with several retinal degeneration disorders. The ABCA13 gene maps to chromosome 7p12.3, a region that contains an inherited disorder affecting the pancreas (Shwachman-Diamond syndrome) as well as a locus involved in T-cell tumor invasion and metastasis (INM7), and therefore is a positional candidate for these pathologies.
Introduction
The ATP‐binding cassette (ABC) gene superfamily encodes a series of transporter proteins that move a wide variety of substances across extra‐ and intracellular membranes. Forty‐eight known human ABC genes can be divided into seven phylogenetically distinct subfamilies. The ABCA gene subfamily is found exclusively in multicellular eukaryotes.
Results
We report here on a unique tandem array of five ABCA genes on chromosome 17q24 defining a phylogenetically distinct group. This is the largest cluster of mammalian ABC genes described to date. They are arranged head‐to‐tail and have similar intron/exon organization in both mouse and human. Northern analysis reveals a heterogeneous pattern of expression in human tissues, with ABCA5 and ABCA10 expressed in skeletal muscle, ABCA6 in the liver, ABCA9 in the heart, and ABCA8 in ovaries. This suggests that these proteins have distinct functions.
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