Analysis of mRNA Partitioning Between the Cytosol and Endoplasmic Reticulum Compartments of Mammalian Cells

All eukaryotic cells display a dramatic partitioning of mRNAs between the cytosol and endoplasmic reticulum (ER) compartments—mRNAs encoding secretory and integral membrane proteins are highly enriched on ER-bound ribosomes and mRNAs encoding cytoplasmic/nucleoplasmic proteins are enriched on cytosolic ribosomes. In current views, this partitioning phenomenon occurs through positive selection—mRNAs encoding signal sequence-bearing proteins are directed into the signal recognition particle pathway early in translation and trafficked as mRNA/ribosome/nascent polypeptide chain complexes to the ER. In the absence of an encoded signal sequence, mRNAs undergo continued translation on cytosolic ribosomes. Recent genome-wide analyses of mRNA partitioning between the cytosol and the ER compartments have identified subsets of mRNAs that are non-canonically partitioned to the ER—although lacking an encoded signal sequence, they are translated on ER-bound ribosomes. These findings suggest that multiple, and as yet unidentified, pathways exist for directing mRNA partitioning in the cell. In this contribution, we briefly review the literature describing the subcellular partitioning patterns of mRNAs and present a detailed methodology for studying this fundamental, yet poorly understood process.