Identification of mitochondrial RNA granules assembly factors and regulators using an image-based siRNA screen
Zaganelli Sofia1, Dimitri Moreau2, Martinou Jean-Claude1
1Department of Cell Biology, University of Geneva, 30 quai Ernest-Ansermet, 1211 Genève 4, Switzerland; 2ACCESS Geneva, University of Geneva, 30 quai Ernest-Ansermet, 1211 Genève 4, Switzerland
Mitochondrial gene expression is essential for mitochondrial respiration, and in humans is entirely dependent on imported nuclear-encoded proteins. The mitochondrial genome itself encodes 13 key subunits of the oxidative phosphorylation system as well as the full complement of tRNAs and ribosomal RNAs, required for their translation within the mitochondrion. Most, if not all the steps of mitochondrial gene expression appear to be spatially organized into two distinct but interacting structures: the nucleoids, which contain the mitochondrial DNA (mt-DNA); and the more recently described mitochondrial RNA granules (MRGs), which contain newly synthesized RNAs and several proteins. From our growing knowledge of specific proteins associated with MRGs, several functions can be ascribed to these structures including RNA processing, RNA maturation, and ribosome assembly. However, in order to advance our understanding of the relevance of MRGs in mitochondrial gene expression, it is essential to understand the as yet unknown mechanisms underlying MRG assembly and regulation, and the interplay between mitochondrial and nuclear gene expression. This is the goal of the present study. We present an automated microscopy-based RNAi screen to identify proteins participating both in the assembly and regulation of MRGs. Applying a multi-parameter phenotypic analysis, we identified a subset of about 60 different nuclear genes involved in controlling the number, morphology and spatial distribution of MRGs within the mitochondrial network. From these candidate genes, we have focused our attention mainly on genes encoding RNA-binding proteins with currently uncharacterized mitochondrial function. Our data are providing fresh insight into the regulation and spatial organization of mitochondrial gene expression.
