Prion protein and RNA: a view from the cytoplasm.

Since it was posited that a cytoplasmic isoform of PrP may be involved in prion diseases, controversies about the isoform's biogenesis and function have emerged in the literature. While the existence of cytoplasmic PrP in vivo and in different cell cultures systems has now been well-established, whether it has specific activity remains unknown. This review outlines recent evidence about the molecular activity of cytoplasmic PrP. Cytoplasmic PrP inhibits a normal cellular stress response by preventing the assembly of protective mRNA stress granules and the synthesis of heat-shock protein 70 following environmental stress. Interference with the stress response correlates with the coalescence of mRNAs in a large cytoplasmic ribonucleoprotein particle. This particle shares similarities with the chromatoid body, a particle that organizes and controls RNA processing in mammalian germ cells as well as in neurons and stem cells from planarians with high regenerative abilities.