P bodies: at the crossroads of post-transcriptional pathways

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Abstract
P bodies are discrete cytoplasmic domains in which proteins that are involved in mRNA degradation, translational repression, mRNA surveillance and RNA-mediated gene silencing colocalize. Key features of P bodies have been conserved throughout evolution. First, P-body assembly is dependent on RNA — treating cells with ribonucleases leads to the dispersion of P bodies. Second, the size and number of P bodies depends on the fraction of mRNAs that are undergoing decapping. Blocking mRNA decay at an early stage (for example, by preventing deadenylation) leads to P-body loss, whereas blocking decapping leads to an increase in the size and number of P bodies. Last, P bodies are dispersed by drugs that stabilize polysomes (for example, cycloheximide) and are enhanced by drugs that release ribosomes from mRNA (for example, puromycin). Therefore, mRNAs must exit the translation cycle to enter into P bodies. The available evidence indicates that mRNA decay, translational repression and mRNA silencing can take place in P bodies. However, it is still unclear whether the integrity of P bodies is required for these processes to occur or whether P bodies arise as a consequence of these activities. P bodies share common components with other messenger ribonucleoprotein (mRNP) granules that are present in stressed mammalian cells (stress granules), in polarized cells such as neurons, and during oogenesis in diverse organisms (polar granules).