ATP-binding cassette transporter A1 contains an NH2-terminal signal anchorsequence that translocates the protein's first hydrophilic domain to the exoplasmic space

Mutations in the ATP-binding cassette transporter A1 (ABCA1) transporter ar e associated with Tangier disease and a defect in cellular cholesterol effl ux, The amino terminus of the ABCA1 transporter has two putative in-frame t ranslation initiation sites, 60 amino acids apart. A cluster of hydrophobic amino acids form a potentially cleavable signal sequence in this 60-residu e extension. We investigated the functional role of this extension and foun d that it was required for stable protein expression of transporter constru cts containing any downstream transmembrane domains. The extension directed transporter translocation across the ER membrane with an orientation that resulted in glycosylation of amino acids immediately distal to the signal s equence. Neither the native signal sequence nor a green fluorescent protein tag, fused at the amino terminus, was cleaved from ABCA1. The green fluore scent protein fusion protein had efflux activity comparable with wild type ABCA1 and demonstrated a predominantly plasma membrane distribution in tran sfected cells. These data establish a requirement for the upstream 60 amino acids of ABCA1. This region contains an uncleaved signal anchor sequence t hat positions the amino terminus in a type II orientation leading to the ex tracellular presentation of an similar to 600-amino acid loop in which loss -of-function mutations cluster in Tangier disease patients.