Nucleocytoplasmic transport determines the biophysical properties in the nucleus.

Abstract

Nucleocytoplasmic transport (NCT) regulates the transport of RNA and proteins through the nuclear envelope to maintain cellular functions and homeostasis in eukaryotic cells. RNA and proteins are actively trafficked through the nuclear pore complex (NPC) with nuclear transport receptors (NTR) as chaperones. NCT controls nuclear volume and distribution of macromolecules, thus affecting the molecular crowding in the nucleus. Molecular crowding is the volume occupied by macromolecules; this biophysical property impacts molecular motion and assembly. However, how the balance of NCT defines the biophysical properties in the nucleus remains unknown. To probe nuclear biophysical properties, we developed a microrheology tool, nuclear Genetically Encoded Multimeric nanoparticles (nucGEMs). nucGEMs are self-assembling, inert fluorescent particles that can probe nuclear biophysical properties with high throughput single particle tracking. In budding yeast S. cerevisiae, conditional depletion of an mRNA export receptor using the anchor-away system significantly decreased the diffusivity of nucGEMs. This result suggests that the inhibition of mRNA export increases nuclear crowding, potentially as a result of mRNA accumulation inside the nucleus. Interestingly, perturbing protein transports led to different effects still under investigation. Inhibiting a major protein export receptor increases nucGEM diffusivity, while inhibiting the import of proteins carrying classical nuclear localization signals (cNLS) did not change the diffusivity of nucGEMs. We hypothesize that perturbations of protein transports lead to potential feedback mechanisms, including changes in the nuclear volume and transcription activities, and/or alterations of NPC permeabilization maintained by NTRs. Together, our results suggest that nuclear export perturbations in budding yeast affect the biophysical properties of the nucleus. Studies investigating the underlying mechanisms are ongoing in both budding yeast and mammalian cells. Understanding how NCT affects the biophysical properties in the nucleus could provide insights into how cells efficiently integrate physically separated nucleus and cytosol.