Hoyle et al. now report that many of the backed-up untranslated mRNAs hang out in a new type of cytoplasmic granule called EGP bodies.
The stress of glucose starvation in yeast abruptly slows translation. So Hoyle et al. wondered what happens to the stalled translation machinery. They discovered that three translation factors, eIF4E, eIF4G, and Pab1p (E, G, and P), huddled together into four or five cytoplasmic foci. The binding of E, G, and P to untranslated mRNAs was thought to commit the transcripts to translation. But when the translation factors formed quiescent foci, the mRNAs went with them.
Two or three of the foci contained an mRNA-decapping enzyme that is found in P bodies—sites implicated in transcript decay. The accompanying mRNAs entering these foci might therefore be degraded. The remaining granules that the authors define as EGP bodies, due to their lack of decapper, might instead be sites of mRNA storage. Already bound to E, G and P, the mRNAs would be ready for translation once the tough times are over, say the authors.
Upon glucose starvation, yeast cells start metabolizing ethanol. This switch requires some translation to produce ethanol-metabolizing enzymes. The team is now trying to figure out how the cell decides which mRNAs go to P bodies for possible degradation, which go to EGP bodies for possible storage, and which continue to be translated.