Alt-RPL36 downregulates the PI3K-AKT-mTOR signaling pathway by interacting with TMEM24
Xiongwen Cao, Alexandra Khitun, Yang Luo, Zhenkun Na, Thitima Phoodokmai, Khomkrit Sappakhaw, Elizabeth Olatunji, Chayasith Uttamapinant, Sarah A. Slavoff
Abstract
Abstract Thousands of human small and alternative open reading frames (smORFs and alt-ORFs, respectively) have recently been annotated. Many alt-ORFs are co-encoded with canonical proteins in multicistronic configurations, but few of their functions are known. Here, we report the detection of alt-RPL36, a protein co-encoded with human RPL36. Alt-RPL36 partially localizes to the endoplasmic reticulum, where it interacts with TMEM24, which transports the phosphatidylinositol 4,5-bisphosphate (PI(4,5)P 2 ) precursor phosphatidylinositol from the endoplasmic reticulum to the plasma membrane. Knock-out of alt-RPL36 increases plasma membrane PI(4,5)P 2 levels, upregulates PI3K-AKT-mTOR signaling, and increases cell size. Alt-RPL36 contains four phosphoserine residues, point mutations of which abolish interaction with TMEM24 and, consequently, alt-RPL36 effects on PI3K signaling and cell size. These results implicate alt-RPL36 as an upstream regulator of PI3K-AKT-mTOR signaling. More broadly, the RPL36 transcript encodes two sequence-independent polypeptides that co-regulate translation via different molecular mechanisms, expanding our knowledge of multicistronic human gene functions.