RNA-binding protein (RBP) recruitment, occupancy, and stoichiometry in nuclear mRNA-protein complexes.
RNA-binding proteins (RBPs) interact with mRNA and associated factors to form supramolecular complexes called messenger ribonucleoprotein (mRNP) particles. These dynamic assemblies change to direct and regulate individual steps of gene expression; however, the composition and functional importance of individual mRNPs remains largely unknown. To address this gap in knowledge, our recent efforts using Saccharomyces cerevisiae have focused on (1) single molecule imaging of isolated complexes to investigate stoichiometry and co-occupancy of RBPs within single mRNPs and (2) live cell imaging of RBP co-transcriptional recruitment to inform mechanisms of mRNP assembly. Our unpublished results demonstrate single mRNP compositional heterogeneity, mRNP plasticity across growth conditions, RBP stoichiometry biased by transcript length and secondary structure, and an order of RBP recruitment during transcription that differs from current models of mRNP assembly. We further demonstrate that depletion of a conserved RBP, with the earliest co-transcriptional arrival time and largest variation in mRNP stoichiometry (1-10 copies), resulted in decreased compaction of nuclear mRNPs. These data identify gene and condition dependent mRNP compositions and provide a quantitative framework for understanding how mRNP composition is functionally linked to biogenesis, packaging, and export.