The University of Texas MD Anderson Cancer Center UTHealth Graduate School of Biomedical Sciences Dissertations and Theses (Open Access)
Date of Graduation
Genes and Development
Doctor of Philosophy (PhD)
Rumi is a protein O-glucosyltransferase that adds the sugar O-glucose onto the serine in the target sequence C-S-X-S-(P/A)-C found within properly folded EGF repeats. It was first discovered to modify the Drosophila Notch extracellular domain and to be required for Notch signaling in a temperature dependent manner, but other targets of Rumi remained unknown. Several other proteins in the Drosophila proteome harbor multiple consensus sequence highly predictive of O-glucose, including the transmembrane protein Crumbs and the secreted protein Eyes shut (Eys). Both of these proteins are required for proper eye development and mutations in their human homologs cause a blindness disorder named retinitis pigmentosa. Therefore, we sought to determine whether Rumi plays a role in photoreceptor development. We found that rumi–/– animals have defects in photoreceptor spacing in which many neighboring rhabdomeres are attached. This phenotype cannot be explained by the loss of O-glucose on Notch or Crumbs. However, eys genetically interacts with rumi, and in rumi–/– animals at the start of rhabdomere separation, Eys accumulates intracellularly and decreased levels of Eys reach the extracellular space. Overexpressing a mutant Eys transgene which contains no intact O-glucosylation sites also results in intracellular accumulation of Eys, suggesting that loss of O-glucose from Eys is the cause. Additionally, both the intracellular accumulation and the rhabdomere attachment defect grow more severe at higher temperatures, and Eys degrades at higher temperatures in rumi–/–. In addition, removing one copy of the chaperone Hsc70-3 enhances the rumi–/– phenotype. Together, these data suggest that loss of O-glucose from Eys causes a defect in its proper folding, which leads to decreased Eys reaching the extracellular space and therefore a failure in full separation of the rhabdomeres.
Rumi, glycosylation, O-glucosylation, photoreceptor, Eyes shut, Eys