Identification of Protor as a novel Rictor-binding component of mTOR complex-2

TLDR

mTOR is a key regulator of cell growth, functioning in two complexes: mTORC1 (mTOR–Raptor–mLST8) that phosphorylates p70S6K at Thr389, and mTORC2 (mTOR–Rictor–mLST8–Sin1) that phosphorylates PKB at Ser473. This study identifies the widely expressed proteins Protor‑1 and Protor‑2 as novel components that bind Rictor within mTORC2. Co‑immunoprecipitation shows Protor‑1/2 associate with other mTORC2 subunits but not Raptor, detergents dissociate mTOR and mLST8 from the Protor‑1–Sin1–Rictor complex, Rictor regulates Protor‑1 expression, and Protor‑1 is not required for assembly of other mTORC2 subunits, establishing it as a new Rictor‑binding subunit whose functional role remains to be defined.

Abstract

The mTOR (mammalian target of rapamycin) protein kinase is an important regulator of cell growth. Two complexes of mTOR have been identified: complex 1, consisting of mTOR–Raptor (regulatory associated protein of mTOR)–mLST8 (termed mTORC1), and complex 2, comprising mTOR–Rictor (rapamycininsensitive companion of mTOR)–mLST8–Sin1 (termed mTORC2). mTORC1 phosphorylates the p70 ribosomal S6K (S6 kinase) at its hydrophobic motif (Thr389), whereas mTORC2 phosphorylates PKB (protein kinase B) at its hydrophobic motif (Ser473). In the present study, we report that widely expressed isoforms of unstudied proteins termed Protor-1 (protein observed with Rictor-1) and Protor-2 interact with Rictor and are components of mTORC2. We demonstrate that immunoprecipitation of Protor-1 or Protor-2 results in the co-immunoprecipitation of other mTORC2 subunits, but not Raptor, a specific component of mTORC1. We show that detergents such as Triton X-100 or n-octylglucoside dissociate mTOR and mLST8 from a complex of Protor-1, Sin1 and Rictor. We also provide evidence that Rictor regulates the expression of Protor-1, and that Protor-1 is not required for the assembly of other mTORC2 subunits into a complex. Protor-1 is a novel Rictor-binding subunit of mTORC2, but further work is required to establish its role.