The Memory Gene, Murashka, Is a Regulator of Notch Signalling and Controls the Size of the Drosophila Germline Stem Cell Niche

We identified Murashka, a RING finger protein, in an oogenesis screen as a regulator of <i>Drosophila</i> ovary germline stem cell niche development. Mutant alleles of <i>murashka</i> exhibited an enlarged niche phenotype reminiscent of increased Notch signalling and displayed genetic interactions with <i>Notch</i> alleles, and with <i>polychaetoid</i>, a regulator of Notch during niche development. These interactions uncovered both positive and negative impacts on Notch in different genetic backgrounds. In S2 cells, Murashka formed a complex with Notch and colocalised with Notch in the secretory pathway. Murashka expression in S2 cells down-regulated Notch signalling levels but could result in increased fold induction due to the proportionally greater decrease in basal ligand-independent activity. In vivo Murashka expression had different outcomes on different Notch target genes. We observed a decrease in the expression of <i>vestigial</i> along the anterior/posterior boundary of the wing imaginal disc, but not of <i>wingless</i> at the dorsal/ventral boundary. Instead, weak ectopic <i>wingless</i> was observed, which was synergistically increased by the coexpression of Deltex, a positive regulator of ligand-independent signalling. Our results identify a novel developmental role for <i>murashka</i>, a gene previously only associated with a function in long-term memory, and indicate a regulatory role for Murashka through a physical interaction with Notch that has context-dependent outcomes. Murashka adds to a growing number of ubiquitin ligase regulators which interact with Notch at different locations within its secretory and endocytic trafficking pathways.