Endoplasmic Reticulum and Golgi Localization Sequences for Mammalian Target of Rapamycin

Abstract

Mammalian target of rapamycin (mTOR) forms two complexes, mTORC1 and mTORC2, that play central roles in cell growth and functions. Only mTORC1 is directly inhibited by the immunosuppressive drug rapamycin. Despite recent progress in identifying new components and functions of the mTOR pathway, relatively little is known about the spatial arrangement of mTOR signaling and the underlying mechanisms. In a previous study, we showed that a large proportion of mTOR is localized to the endoplasmic reticulum (ER) and Golgi in many common cell lines. Here, we report the identification of an internal mTOR sequence that contains two HEAT (HT) repeats, HT18 and HT19, and two intervening interunit spacers (IUSs), IUS17 and IUS18, which is sufficient to target enhanced green fluorescent protein to the Golgi. Surprisingly, deletion of IUS17 from this Golgi localization sequence (GLS) converts it to an ER localization sequence (ELS). Deletion of HT19, a common element of both GLS and ELS from the full-length mTOR, causes delocalization of mTOR and inhibits the ability of mTOR to promote S6 phosphorylation. Moreover, overexpression of GLS and ELS inhibits both mTOR complexes. Together, our results reveal unusual ER- and Golgi-targeting sequences and suggest that anchoring to these organelles is important for the functions of mTOR complexes.