A mutant cytochrome b(5) with a lengthened membrane anchor escapes from the endoplasmic reticulum and reaches the plasma membrane

Many resident membrane proteins of the endoplasmic reticulum (ER) do not have known retrieval sequences. Among these are the so-called tail-anchored proteins, which are bound to membranes by a hydrophobic tail close to the C terminus and have most of their sequence as a cytosolically exposed N-terminal domain. Because ER tail-anchored proteins generally have short (less than or equal to 17 residues) hydrophobic domains, we tested whether this feature is important for localization, using cytochrome b(5) as a model. The hydrophobic domain of cytochrome b(5) was lengthened by insertion of five amino acids (ILAAV), and the localization of the mutant was analyzed by immunofluorescence in transiently transfected mammalian cells. While the wild-type cytochrome was localized to the ER, the mutant was relocated to the surface. This relocation was not due to the specific sequence introduced, as demonstrated by the ER localization of a second mutant, in which the original length of the membrane anchor was restored, while maintaining the inserted ILAAV sequence. Experiments with brefeldin A and with cycloheximide demonstrated that the extended anchor mutant reached the plasma membrane by transport along the secretory pathway. We conclude that the short membrane anchor of cytochrome b(5) is important for its ER residency, and we discuss the relevance of this finding for other ER tail-anchored proteins.