The KH-domain protein Psi fine-tunes transcription of developmental genes to pattern cell and tissue growth in Drosophila

© 2019 Olga ZaytsevaMammalian FUBP-family proteins play roles in both transcription and RNA processing, by binding single stranded nucleic acids via their KH domains. Particularly, in vitro mammalian tissue culture studies suggest that FUBP1 drives MYC transcription in response to growth stimuli. MYC is a key regulator of growth and cell cycle progression essential for normal development. Furthermore, expression of MYC is aberrantly upregulated in approximately 70% of all cancers. Understanding the mechanisms that control MYC expression may lead to insights into how MYC dysregulated in diseases such as cancer. However, the significance of the transcriptional control by FUBP1 proteins at the MYC promoter and genome-wide has remained unclear in the context of development in whole organisms. Mammalian studies have been complicated by the fact that mammals have three FUBP-family proteins that bind overlapping genomic targets. However, in Drosophila, the three mammalian FUBP proteins are represented by one ortholog, Psi. Due to a high degree of structural similarity between FUBP1 and Psi proteins, we hypothesized that Psi will also regulate transcription of MYC and other targets. We have taken advantage of the reduced functional redundancy in Drosophila to demonstrate that an essential in vivo function of Psi is control of cell and tissue growth. Analysis of published Co-IP-mass spectrometry screens positioned Psi in an interactome predominantly comprised of RNA Polymerase II transcriptional machinery, particularly the transcriptional Mediator (MED) complex, a known sensor of developmental signalling inputs. Our work indicates that a key transcriptional target of the Psi/MED network that impacts tissue growth is Myc. During development of the wing disc epithelial tissue, Psi was required to promote Myc expression in response to activated PI3K pathway signalling. In addition to Myc, Psi binds to multiple transcriptional targets in the wing epithelium in vivo. Intersection of RNA-seq to detect differentially expressed genes following Psi depletion, and wing-specific direct binding sites identified genome-wide by Targeted DamID, revealed a developmental patterning signature for Psi including Wnt, Notch and TGF-beta cell fate determinant pathways. Moreover, analysis of direct Psi targets identified several novel growth regulators, including the TGF-beta signalling regulator tolkin (tok) and the CD36 scavenger receptor-related epithelial membrane protein (emp). Together our data suggest Psi integrates cellular signalling inputs to directly modulate transcriptional outputs and fine-tune development