Cellular stress attenuates growth factor signaling through a phosphorylation event that blocks substrate access to the kinase complex mTORC2.

Growth factor signaling coupled to activation of the phosphatidylinositol-3-OH kinase (PI3K)/Akt pathway plays a crucial role in the regulation of cell proliferation and survival. The key regulatory kinase of Akt has been identified as mammalian target of rapamycin complex 2 (mTORC2), which functions as the PI3K-dependent Ser-473 kinase of Akt. This kinase complex is assembled by mTOR and its essential components rictor, Sin1 and mLST8. The recent genetic screening study in Caenorhabditis elegans has linked a specific point mutation of rictor to an elevated storage of fatty acids that resembles the rictor deficiency phenotype. In our study, we show that in mammalian cells the analogous single rictor point mutation (G934E) prevents the binding of rictor to Sin1 and the assembly of mTORC2, but this mutation does not interfere with the binding of the rictor-interacting protein Protor. A substitution of the rictor Gly-934 residue to a charged amino acid prevents formation of the rictor/Sin1 heterodimer. The cells expressing the rictor G934E mutant remain deficient in the mTORC2 signaling, as detected by the reduced phosphorylation of Akt on Ser-473 and a low cell proliferation rate. Thus, although a full length of rictor is required to interact with its binding partner Sin1, a single amino acid of rictor Gly-934 controls its interaction with Sin1 and assembly of mTORC2.

The H3.3 histone variant has been a subject of increasing interest in the field of chromatin studies due to its two distinguishing features. First, its incorporation into chromatin is replication independent unlike the replication-coupled deposition of its canonical counterparts H3.1/2. Second, H3.3 has been consistently associated with an active state of chromatin. In accordance, this histone variant should be expected to be causally involved in the regulation of gene expression, or more generally, its incorporation should have downstream consequences for the structure and function of chromatin. This, however, leads to an apparent paradox: In cells that slowly replicate in the organism, H3.3 will accumulate with time, opening the way to aberrant effects on heterochromatin. Here, we review the indications that H3.3 is expected both to be incorporated in the heterochromatin of slowly replicating cells and to retain its functional downstream effects. Implications for organismal aging are discussed.

Bow T. Shrike1, Gustav Nilsonne2, Vincent Weisser1, Rakhan Aimbetov1,1 VitaDAO 2 Karolinska Institutet, Stockholm, Sweden To whom correspondence should be addressed; r.aimbetov@gmail.comAbstractOne ma...