The RNA-binding protein AUF1 facilitates Akt phosphorylation at the membrane

Mammalian target of rapamycin (mTOR), which is part of mTOR complex 1 (mTORC1) and mTORC2, controls cellular metabolism in response to levels of nutrients and other growth signals. A hallmark ofmTORC2 activation is the phosphorylation of Akt, which becomes upregulated in cancer. How mTORC2 modulates Akt phosphorylation remains poorly understood. Here, we found that the RNA-binding protein, AUF1 (ARE/poly(U)-binding/degradation factor 1), modulates mTORC2/Akt signaling. We determined that AUF1 is required for phosphorylation of Akt at Thr308, Thr450, and Ser473 and that AUF1 also mediates phosphorylation of the mTORC2-modulated metabolic enzyme glutamine fructose-6-phosphate amidotransferase 1 at Ser243. In addition, AUF1 immunoprecipitation followed by quantitative RT-PCR revealed that themRNAs of Akt, glutamine fructose-6-phosphate amidotransferase 1, and the mTORC2 component SIN1 associate with AUF1. Furthermore, expression of the p40 and p45, but not the p37 or p42, isoforms of AUF1 specificallymediateAkt phosphorylation. In the absence ofAUF1, subcellular fractionation indicated that Akt fails to localize to the membrane. However, ectopic expression of amembrane-targeted allele of Akt is sufficient to allow Akt-Ser473 phosphorylation despite AUF1 depletion. Finally, conditions that enhance mTORC2signaling, such as acute glutaminewithdrawal, augment AUF1 phosphorylation, whereas mTOR inhibition abolishes AUF1 phosphorylation. Our findings unravel a role for AUF1 in promoting membrane localization of Akt to facilitate its phosphorylation on this cellular compartment. Targeting AUF1 could have therapeutic benefit for cancers with upregulated mTORC2/Akt signaling.

Automated summary

The RNA-binding protein AUF1 plays a crucial role in modulating mTORC2/Akt signaling by promoting membrane localization of Akt and facilitating its phosphorylation. AUF1 also mediates the phosphorylation of the mTORC2-regulated metabolic enzyme glutamine fructose-6-phosphate amidotransferase 1. These findings have significant implications for the therapeutic targeting of cancers with upregulated mTORC2/Akt signaling.