Single mRNP Analysis Reveals that Small Cytoplasmic mRNP Granules Represent mRNA Singletons

Small cytoplasmic mRNP granules are implicated in mRNA transport, translational control, and decay. Using super-resolution microscopy and fluorescence correlation spectroscopy, we analyzed the molecular composition and dynamics of single cytoplasmic YBX1_IMP1 mRNP granules in live cells. Granules appeared elongated and branched, with patches of IMP1 and YBX1 distributed along mRNA, reflecting the attachment of the two RNA-binding proteins in cis. Particles form at the nuclear pore and do not associate with translating ribosomes, so the mRNP is a repository for mRNAs awaiting translation. In agreement with the average number of mRNA-binding sites derived from crosslinked immunoprecipitation (CLIP) analyses, individual mRNPs contain 5–15 molecules of YBX1 and IMP1 and a single poly(A) tail identified by PABPC1. Taken together, we conclude that small cytoplasmic mRNP granules are mRNA singletons, thus depicting the cellular transcriptome. Consequently, expression of functionally related mRNAs in RNA regulons is unlikely to result from coordinated assembly. [Display omitted] •Cytoplasmic YBX1_IMP1 mRNP granules are elongated and branched structures•mRNPs are formed at the nuclear pore and await translation•Small cytoplasmic IMP1 and YBX1 mRNPs are mRNA singletons Using super-resolution microscopy and fluorescence correlation spectroscopy, Mateu-Regué et al. analyze the composition of single cytoplasmic YBX1_IMP1 mRNP granules in live cells. mRNPs are found to represent mRNA singletons depicting the cellular transcriptome. Consequently, mRNA regulons are unlikely to result from coordinated assembly.