Human ABCA1 contains a large amino-terminal extracellular domain homologous to an epitope of Sjogren's syndrome

ABCA1 has been suggested to play a key role in cellular lipid release from peripheral cells. In order to study structure-function relationship of this protein, the protein product of a full-length human ABCA1 cDNA was examine d for its functions and topological orientation. The electrophoretic mobili ties of human ABCA1 expressed in transfected cells increased when treated w ith N-glycosidase F, suggesting that ABCA1 is highly glycosylated, The ABCA 1 was photoaffinity-labeled with ATP and mediated the apoA-I-dependent-rele ase of cholesterol and phospholipid. The influenza hemagglutinin (HA) epito pe was introduced into the amino-terminus (N-HA) or between the residues 20 7 and 208 (207-HA) of the protein. While an antibody against the C-terminus peptide of ABCA1 detected both fusion proteins, an anti-HA antibody did no t react with the N-HA fusion protein. Confocal microscopy demonstrated stro ng cell surface signal with the anti-HA antibody of nonpermeabilized HEK293 cells expressing the 207-HA fusion protein. The results suggested that the signal peptide in the amino-terminal region is cleaved off in its mature f orm and that the following large hydrophilic region is exposed to outside o f cells unlike previously proposed models. We found that this amino-termina l extracellular domain contains a segment homologous to the autoantigen SS- N, an epitope of Sjogren's syndrome, and further identified that ABCA7 code s for the autoantigen SS-N, (C) 2001 Academic Press.