RNA to be degraded (red) colocalizes with degradation factors (green) in foci.

Cytoplasmic structures are sites of active mammalian mRNA decay, according to Cougot et al. (page 31).

The group had previously identified cytoplasmic foci that included two human mRNA decapping enzymes. They now add subunits of a deadenylase, exonuclease, and possible helicase to the list of proteins found at these sites. After inhibition of a 5′–3′ exonuclease, poly(A)+ RNA accumulates at the same sites, further suggesting that these are locations for active degradation of RNA rather than passive storage centers for degradation factors. The foci almost completely disappear after addition of either translational inhibitors that are known to stabilize RNAs or transcriptional inhibitors that deplete the cytoplasm of all mRNA.

Similar structures have been seen in yeast, although these structures were fewer in number and only visible under certain nutrient conditions. Both findings suggest that the cytoplasm is more structured than previously thought. The regulation of foci formation remains a subject for future studies. ▪