Abstract

Stress granules (SGs) and processing bodies (PBs) are membrane-less RNP granules that assemble in response to stress. They form through liquid-liquid phase separation that is driven by high local concentrations of key proteins and RNAs to form condensates. PBs were originally hypothesised to be sites of cellular mRNA decay. However, PBs have recently been proposed to also act as storage sites for translationally repressed mRNAs that can exit PBs and re-initiate translation. These two functions are not mutually exclusive and hence this current project aims to characterize the fate of mRNAs that localize to PBs, focussing on what determines whether different mRNAs are destined for degradation versus storage. We will use a non-invasive metabolic labeling method to precisely measure the decay kinetics of mRNAs localized to PBs relative to their cytosolic counterparts. This will enable us to test the hypothesis that PBs provide a specialized microenvironment where certain mRNAs are degraded while others are earmarked for storage as part of an adaptive response to stress. We will define the cis-acting features that determine the individual fate of mRNAs in PBs as well as the associated factors that mediate it by combining genetics with advanced microscopy approaches to follow PB-localized mRNAs in mutants. The molecular mechanisms by which mRNAs are protected from enzymatic degradation within PBs are unclear and we will examine the requirement for recently identified PB proteins in mRNA stability. We will test the hypothesis that PB and SG formation is a continuum and that mRNAs initially stored in PBs can be passed onto SGs for longer term storage during prolonged stress conditions. Together, our studies will elucidate the functional roles of PBs in RNA metabolism and how these specialized microenvironments moderate gene expression to coordinate the cellular outcomes in response stress.