Abstract
The Akt kinase is a critical effector in growth factor signaling. Activation of Akt driven by the growth factor dependent PI3K (phosphatidylinositol-3-OH kinase) is coupled to the plasma membrane translocation and phosphorylation of Akt on two sites by PDK1 (phosphoinositide-dependent protein kinase-1) on Thr-308 and by mTORC2 (mammalian Target of Rapamycin Complex 2) on Ser-473. In our study we examined the sub-cellular localization of mTORC2 and identified that this kinase complex predominantly resides on endoplasmic reticulum (ER). Our immunostaining analysis did not show a substantial co-localization of the mTORC2 component rictor with Golgi, lysosome, clathrin-coated vesicles, early endosomes, or plasma membrane but indicated a strong co-localization of rictor with ribosomal protein S6 and ER marker. Our biochemical study also identified the mTORC2 components rictor, SIN1, and mTOR as the highly abundant proteins in the ER fraction, whereas only small amount of these proteins are detected in the plasma membrane and cytosolic fractions. We found that growth factor signaling does not alter the ER localization of mTORC2 and also does not induce its translocation to the plasma membrane. Based on our study we suggest that the mTORC2-dependent phosphorylation of Akt on Ser-473 takes place on the surface of ER.