Cytoplasmic mRNP granules at a glance.

Introduction From their transcriptional birth to their degradation, cellular mRNAs are coated with proteins in messenger ribonucleoprotein (mRNP) complexes. The mRNP composition controls every aspect of the life of the mRNA, from pre-mRNA processing to mRNA localization, translation and turnover. Transitions between these events are accompanied by major mRNP remodeling and exchange of mRNP proteins. Upon entering the cytoplasm, the mRNP composition dictates whether the mRNA engages in translation, or remains translationally inactive and is subject to either storage or degradation (see Poster). In recent years, it has become clear that many translationally inactive mRNPs have the ability to assemble into cytoplasmic mRNP granules (for reviews, see Anderson and Kedersha, 2009; Arkov and Ramos, 2010; Buchan and Parker, 2009; Eulalio et al., 2007a; Franks and LykkeAndersen, 2008; Kulkarni et al., 2010; Zeitelhofer et al., 2008). The best-characterized mRNP granules in the somatic cell cytoplasm are processing bodies (PBs) and stress granules (SGs). To outline the current understanding of cytoplasmic mRNP granules, we will discuss the protein complexes required for the assembly of mRNPs into PBs and SGs, the conditions under which assembly occurs and the potential outcomes of assembling mRNPs into large macromolecular complexes. This discussion is relevant also to other cytoplasmic mRNP granules, for example those found in germ cells and neuronal cells, and during early development (Anderson and Kedersha, 2009; Arkov and Ramos, 2010; Zeitelhofer et al., 2008), because these mRNP granules probably function in a similar manner to PBs and SGs.