α-Phenylalanyl tRNA synthetase competes with Notch signaling through its N-terminal domain

The alpha subunit of the cytoplasmic Phenylalanyl tRNA synthetase (alpha-PheRS, FARSA in humans) displays cell growth and proliferation activities and its elevated levels can induce cell fate changes and tumor-like phenotypes that are neither dependent on the canonical function of charging tRNA(Phe) with phenylalanine nor on stimulating general translation. In intestinal stem cells of Drosophila midguts, alpha-PheRS levels are naturally slightly elevated and human FARSA mRNA levels are elevated in multiple cancers. In the Drosophila midgut model, elevated alpha-PheRS levels caused the accumulation of many additional proliferating cells resembling intestinal stem cells (ISCs) and enteroblasts (EBs). This phenotype partially resembles the tumor-like phenotype described as Notch RNAi phenotype for the same cells. Genetic interactions between alpha-PheRS and Notch suggest that their activities neutralize each other and that elevated alpha-PheRS levels attenuate Notch signaling when Notch induces differentiation into enterocytes, type II neuroblast stem cell proliferation, or transcription of a Notch reporter. These non-canonical functions all map to the N-terminal part of alpha-PheRS which accumulates naturally in the intestine. This truncated version of alpha-PheRS (alpha-S) also localizes to nuclei and displays weak sequence similarity to the Notch intracellular domain (NICD), suggesting that alpha-S might compete with the NICD for binding to a common target. Supporting this hypothesis, the tryptophan (W) residue reported to be key for the interaction between the NICD and the Su(H) BTD domain is not only conserved in alpha-PheRS and alpha-S, but also essential for attenuating Notch signaling. Author summaryAminoacyl tRNA synthetases charge tRNAs with their cognate amino acid to ensure proper decoding of the genetic code during translation. Independent of its aminoacylation function, the alpha subunit of Drosophila cytoplasmic Phenylalanyl tRNA synthetase (alpha-PheRS, FARSA in humans) has an additional activity that promotes growth and proliferation. Here we describe that elevated alpha-PheRS levels also induce cell fate changes and tumorous phenotypes in Drosophila midguts. Excessive proliferating cells with stem and progenitor cell characteristics accumulate and the composition of the terminally differentiated cells changes, too. This phenotype together with observed genetic interactions between alpha-PheRS and Notch levels show that alpha-PheRS counteracts Notch signaling in many different tissues and developmental stages. This novel activity of alpha-PheRS maps to its N-terminal part, which is naturally produced. The fragment contains a DNA binding domain, translocates into nuclei, and displays essential similarities to a Notch domain that binds to the downstream transcription factor. This suggests that it might be competing with Notch for binding to a common target. Not only because Notch plays important roles in many tumors, but also because FARSA mRNA levels are considerably upregulated in many tumors, this novel activity deserves more attention for cancer research.