A NOVEL TRANSMEMBRANE TRANSPORTER ENCODED BY THE XPCT GENE IN XQ13.2

To study the regulation and chromosomal basis of X chromosome inactivation, we have physically characterized the region in Xq13.2 known to contain the X inactivation center (XIC), a locus required in cis for inactivation to occur. Here, we report a novel gene isolated by positional cloning in this region. The gene (previously identified as DXS128E) encodes a predicted 67 kDa protein containing twelve hydrophobic transmembrane domains, characteristic of a family of transporter proteins. Presence of an N-terminal PEST domain, consisting mainly of proline/glutamic acid repeats, suggests that the protein may be rapidly or conditionally degraded. We designate this gene XPCT for X-linked PEST-containing transporter. Expression studies suggest that XPCT is subject to X chromosome inactivation, being expressed only from the active X, despite mapping within 600 kb of the XIST gene which is expressed exclusively from the inactive X. Thus, a chromosomal switch in inactivation pattern occurs between these two genes on the X chromosome.