One mRNA species (green) is found in both SGs (blue) and PBs (red).

Selected mRNAs and their bound proteins are passed from stress granules (SGs) to processing bodies (PBs), according to evidence presented by Kedersha et al. (page 871). The group suggests that SGs hold and sort the overflow from the translational apparatus, some of which is subsequently sent to PBs for degradation.

Stresses such as heat shock induce the cytoplasmic appearance of SGs—aggregates of mRNAs that are redirected from polysomes when translation initiation is blocked. The new results show that components of SGs sometimes flow to sites of mRNA degradation called PBs. Other proteins are found only at SGs, such as eIF3 and poly(A)-binding proteins, or only at PBs, such as decapping enzymes.

SGs and PBs formed independently—mild heat shock induced only SGs, and PBs formed even in mutant cells in which SGs cannot form. But often their appearance and disappearance were linked, with contents flowing from SGs to PBs. Strong heat shock induced SGs first and transiently; only later did PBs appear. SGs dissolved before PBs when polysome disassembly was blocked.

PBs were often spatially linked to SGs. Expression of the SG protein TTP, which promotes mRNA decay, strengthened their interactions. Linkages are probably a result of shared protein and RNA components: eIF4E, TTP, and mRNA species were found in both sites. More links in the presence of TTP suggest that more RNA is being sent to PBs for destruction. If not sent to PBs, mRNA might move back to polysomes when translation resumes, or simply continue waiting at SGs for cellular conditions to change.

FRAP analysis revealed that some shared components moved quickly in and out of SGs and PBs, whereas others were much slower and might thus be scaffold proteins. The moving proteins are probably those that stay associated with the mRNA as it moves.