Post-translational targeting of a tail-anchored green fluorescent protein to the endolpasmic reticulum.

Microsomal aldehyde dehydrogenase (msALDH) is a tail-anchored protein localized to the cytoplasmic face of the endoplasmic reticulum (ER). The carboxyl-terminal 35 amino acids of msALDH possess ER-targeting sequences in addition to a hydrophobic membrane-spanning domain. To study the mechanism for ER targeting of this protein in vivo, we took advantage of a green fluorescent protein-msALDH fusion protein containing the last 35 amino acids of msALDH [GFPALDH(35)]. When expressed from cDNA in COS-7 cells, the fusion protein was localized to the ER. We then prepared a recombinant fusion protein and injected it into the cytoplasm of COS-7 cells. The injected protein was correctly localized to the ER after a 30-min incubation at 37 degrees C. However, a recombinant fusion protein that contained only the transmembrane domain of msALDH failed to be targeted to the ER. When the assay was carried out at 4 degrees C, the recombinant GFPALDH(35) remained in the cytoplasm. Moreover, incubation of COS-7 cells under conditions of ATP depletion resulted in the cytoplasmic distribution of the injected protein. These results indicate that GFPALDH(35) is targeted to the ER post-translationally via an ATP-dependent pathway. This microinjection system worked effectively in different mammalian cell types, suggesting a common mechanism for ER targeting of the tail-anchored protein.