The amino-terminal 29 amino acids of cytochrome P450 2C1 are sufficient for retention in the endoplasmic reticulum

Cytochromes P450 are inserted into and anchored to the endoplasmic reticulum (ER) membrane by a hydro-phobic signal sequence at the NH₂ terminus. To determine whether the NH₂-terminal sequence might also have an ER retention function, the NH₂-terminal 29 amino acids of cytochrome P450 2C1, with and without an additional 29 amino acids containing an N-glycosylation site, were fused either to a soluble cytoplasmic protein, Escherichia coli β-galactosidase, or to a secreted protein, E. coli alkaline phosphatase, and the hybrid proteins were expressed in COS1 cells. Subcellular fractionation indicated that both the β-galactosidase and alkaline phosphatase hybrid proteins cosedimented with marker enzymes for ER membranes, and localization by immunofluorescent staining was consistent with an ER location. Hybrid proteins with the NH₂-terminal glycosylation site were glycosylated in COS1 cells, and the carbohydrate moiety was sensitive to endoglycosidase H digestion, providing further evidence that the proteins were retained in the ER. In vitro studies of membrane insertion of the alkaline phosphatase hybrid indicated that fusion to alkaline phosphatase did not alter the topological properties of the cytochrome P450 NH2-terminal sequence. In addition, alkaline phosphatase fused to the extracellular and transmembrane domains of epidermal growth factor receptor was transported to the plasma membrane in COS1 cells, which establishes that alkaline phosphatase as a cytoplasmic domain does not prevent transport from the ER. These observations indicate that the large cytoplasmic domain of cytochrome P450 is not required for retention in the ER and suggest that a specific sequence or structure within the NH₂-terminal 29 amino acids functions as an ER retention signal.