Decker et al. now show that a yeast mRNA decapping activator protein contributes to both jobs: it forms part of the building block and also helps to glue those blocks together.
P-bodies are cytoplasmic granules that contain unused mRNAs with their associated proteins (complexes known as mRNPs) and mRNA decay factors. Much of the team's previous work investigated how the mRNPs—which form the building blocks—affect mRNA function. How and why mRNPs assemble into P-bodies was unclear.
The new work further reveals the inner organization of the building blocks by identifying how an mRNA decapping activator, called Edc3p, brings together the mRNP components. Edc3p bound to two protein complexes that associated with P-body–bound mRNAs: a complex of 5′ decapping enzymes and a complex of other decapping activators.
mRNPs then came together into large, visible P-bodies via specific domains of Edc3p, which the team identified through deletion analysis. Removing either of these gluing domains or the entire Edc3p protein resulted in a loss of visible P-bodies from yeast cells.
Yeast unable to assemble large P-bodies just as easily repressed translation and degraded some mRNAs, showing that neither Edc3p or visible P-bodies are required for these functions.
Although the team has yet to identify abnormalities in yeast that are unable to assemble P-bodies, the fact that visible P-bodies are a conserved phenotype of multiple species suggests that aggregation is indeed important. Aggregation might affect specific mRNAs or be needed to prevent mRNPs from interacting with other cellular proteins, say the authors.