mRNAs scan themselves out

In eukaryotic cells, mRNAs are transcribed in the nucleus and must then be exported across the nuclear envelope before they can be translated in the cytoplasm. Multiple factors bind to nuclear mRNAs and mediate their translocation through the nuclear pores, but it is diffi cult to observe the rapid export of individual RNA molecules in real time. Two papers now track the movements of single mRNAs in budding yeast, yielding important new insights into the export process (1, 2). Montreal visualized individual mRNAs by adding multiple PP7-binding sites to different transcripts and expressing them together with GFP-tagged PP7 and an mCherry-tagged nuclear pore protein (1). The mRNAs seemed to " scan " the inner surface of the nuclear envelope before they were translocated to the cytoplasm or released back into the nucleoplasm. RNAs remained at the nuclear periphery for up to a second, apparently moving between neighboring nuclear pore complexes (NPCs). Similar mRNA scanning behaviors have previously been observed in mammalian cells (3). Once they have diffused to the nuclear periphery, RNAs and their associated proteins encounter the nuclear basket, a structure formed by eight protein filaments that protrude from the nuclear side of NPCs. The fi laments are formed by two myosin-like proteins, Mlp1 and Mlp2, and, although the basket isn't required for mRNA export, it appears to have an important quality control function, preventing ribonucleoprotein (RNP) complexes from accessing the pore until they have been remodeled into their mature, export-ready form (4). Saroufi m et al. found that, when they completely removed the baskets by deleting MLP1 and MLP2, mRNAs were quickly released into the nucleoplasm, instead of being retained at the nuclear periphery. The researchers saw a similar effect when they specifically abrogated the interaction between Mlp1's C-terminal domain and the mRNA-binding protein Nab2. " So the nuclear basket is required for this scanning behavior, probably by serving as an interaction platform for RNPs at the nuclear periphery, " Zenklusen explains. " This could allow quality control steps to happen, and help RNAs stay at the periphery until pores become available to translocate them. " Meanwhile, researchers from the labo-Edmonton) discovered that, by removing the yeast cell wall to reduce background fluorescence levels, they could image single, PP7-labeled mRNAs at frame rates quick enough to follow the export event itself (2). Led by Carlas Smith, Azra Lari, and Carina Derrer, the team found that GFA1 transcripts took …