P bodies proliferate when mRNA degradation is blocked (right).


Another day, another organelle. Cytoplasmic foci of various mRNA processing enzymes have now been confirmed by Ujwal Sheth and Roy Parker (University of Arizona, Tucson, AZ) as functional sites for degrading mRNA. But that confirmation has led to a host of questions about the origins and range of functions of these processing bodies (P bodies).

Sheth and Parker found that mRNA decapping enzymes, activators of decapping, and an mRNA nuclease all clustered in two to three P bodies per budding yeast cell. The P bodies melted away when mRNA turnover was inhibited before decapping (by inhibiting deadenylation), but proliferated when the inhibition was at or after the decapping step. Finally, a decay intermediate was localized to P bodies.

The range of reactions occurring in the P bodies remains to be explored, as does the significance of the structure. “Whether the [degradation] biochemistry requires the macroscopic environment [of the P body] or can occur in a dispersed fashion is not known,” says Parker.A full understanding of P body control is also hampered by the field's poor grip on what controls the switches between different mRNA states: from active translation to storage or destruction. At least one factor implicated in storage is found in the P bodies, so these bodies may do double duty in storage and destruction. At the very least, says Parker, the P bodies sequester mRNAs that are being degraded, so they do not compete for factors with mRNAs that are being actively translated. Determining how much additional regulation is present is work for the future. ▪


Sheth, S., and R. Parker.