Recent studies reported that METTL4 regulates DNA 6mA in vivo and therefore is a candidate DNA m6A methyltransfease. However, the enzymatic activity of METTL4 in vitro has not been demonstrated in part due to t he difficulties of obtaining well-folded proteins. Here we show that mettl4 is a major methyltransfase responsible for m6A methylation of U2 snRNA both in vitro and in vivo in fly, and identify adenosine at 29th position as the site of m6A methylation. This study answered a long-standing question regarding the enzymatic activity of METTL4, and thus paved the way for further investigating the functions of METTL4 in different biological settings.

The proto-oncogene YAP /Yki, a transcription co-factor of the Hippo pathway, has been linked to many cancers. YAP interacts with DNA-binding TEAD/Sd proteins to regulate expression of its transcriptional targets. Disruption of YAP-TEAD therefore offers a potential therapeutic strategy. The mammalian Vestigial Like (VGLL) protein, specifically its TONDU domain, has been shown to competitively inhibit YAP-TEAD interaction and a TONDU peptide can suppress YAP-induced cancer. As TONDU could potentially be developed into a therapeutic peptide for multiple cancers, we evaluated its efficacy in Yki-driven adult Intestinal Stem Cell (ISC) tumors in Drosophila. We show that oral uptake of the TONDU peptide is highly effective at inhibiting Yki-driven gut tumors by suppressing YAP-TEAD interaction. Comparative proteomics of early and late stage Yki-driven ISC tumors revealed enrichment of a number of proteins, including members of the integrin signaling pathway, such as Talin, Vinculin and Paxillin. These, in turn displayed a decrease in their levels in TONDU-peptide treated tumors. Further, we show that Sd binds to the regulatory region of integrin-coding gene, mew, which codes for αPS1, a key integrin of the ISCs. In support to a possible role of integrins in Yki-driven ISC tumors, we show that genetic downregulation of mew arrests Yki-driven ISC proliferation, reminiscent of the effects of TONDU peptide. Altogether, our findings present a novel platform for screening therapeutic peptides and provide insights into tumor suppression mechanisms.SIGNIFICANCE STATEMENTDiscovering novel strategies to inhibit oncogene activity is a priority in cancer biology. As signaling pathways are widely conserved between mammals and Drosophila, these questions can be effectively addressed in this model organism. Here, we show that progression of Drosophila Intestinal Stem Cell (ISC) tumors induced by gain of an oncogenic form of the transcription co-factor Yki can be suppressed by feeding a peptide corresponding to the conserved TONDU domain of Vestigial (Vg), which blocks binding of Yki to the Sd transcription factor. Further, we show that down regulation of the integrin signaling pathway is causally linked to TONDU-peptide-mediated ISC tumor suppression. Our findings reveal that Drosophila can be successfully used to screen peptides for their therapeutic applications.