Hairpin orientation of sterol regulatory element-binding protein-2 in cell membranes as determined by protease protection

Abstract

Sterol regulatory element-binding proteins (SREBP-1 and SREBP-2) are proteins of approximately 1150 amino acids each that are attached to membranes of the endoplasmic reticulum (ER). In sterol-depleted cells, a protease releases an NH2-terminal fragment of approximately 500 amino acids that contains a basic helix-loop-helix leucine zipper motif. This fragment enters the nucleus and stimulates transcription of genes encoding the low density lipoprotein receptor and enzymes of cholesterol biosynthesis. Prior evidence indicates that the SREBPs are attached to membranes by virtue of an 80-residue segment located approximately 80 amino acids to the COOH-terminal side of the leucine zipper. This segment contains two long hydrophobic sequences separated by a short hydrophilic sequence of approximately 30 amino acids. We have proposed a hairpin model in which the two hydrophobic sequences span the membrane, separated by the short hydrophilic sequence which projects into the lumen of the ER (the "lumenal loop"). The model predicts that the NH2- and COOH-terminal segments face the cytosol. To test this model, we constructed a cDNA encoding human SREBP-2 with epitope tags at the NH2 terminus and in the lumenal loop. The COOH-terminal region was visualized with a newly developed monoclonal antibody against this region. Sealed membrane vesicles were isolated from cells expressing the epitope-tagged version of SREBP-2. Trypsin treatment of these vesicles destroyed the NH2- and COOH-terminal segments and reduced the lumenal epitope to a size consistent with protection of the lumenal sequence plus the two membrane-spanning segments. The lumenal epitope tag contained two potential sites for N-linked glycosylation. The size of the trypsin-protected fragment was reduced by treatment with N-Glycanase and endoglycosidase H, indicating that this segment was located in the lumen of the ER where it was glycosylated. These data provide strong support for the hairpin model.