The glycosyltransferase ALG3 is an AKT substrate that regulates protein N-glycosylation

The PI3K/AKT signaling pathway is frequently dysregulated in cancer and controls key cellular processes such as survival, proliferation, metabolism and growth. Protein glycosylation is essential for proper protein folding and is also often deregulated in cancer. Cancer cells depend on increased protein folding to sustain oncogene-driven proliferation rates. The N-glycosyltransferase asparagine-linked glycosylation 3 homolog (ALG3), a rate-limiting enzyme during glycan biosynthesis, catalyzes the addition of the first mannose to glycans in an alpha-1,3 linkage. Here we show that ALG3 is phosphorylated downstream of the PI3K/AKT pathway in both growth factor-stimulated cells and PI3K/AKT hyperactive cancer cells. AKT directly phosphorylates ALG3 in the amino terminal region at Ser11/Ser13. CRISPR/Cas9-mediated depletion of ALG3 leads to improper glycan formation and induction of endoplasmic reticulum stress, the unfolded protein response, and impaired cell proliferation. Phosphorylation of ALG3 at Ser11/Ser13 is required for glycosylation of cell surface receptors EGFR, HER3 and E-cadherin. These findings provide a direct link between PI3K/AKT signaling and protein glycosylation in cancer cells.