Unraveling docking and initiation of mRNA export through the nuclear pore complex

Abstract

The nuclear export of mRNA through the nuclear pore complex (NPC) is a process required for the healthy functioning of human cells, making it a critical area of research. However, the geometries of mRNA and the NPC are well below the diffraction limit of light microscopy, thereby presenting significant challenges in evaluating the discrete interactions and dynamics involved in mRNA nuclear export through the native NPC. Recent advances in biotechnology and single-molecule super-resolution light microscopy have enabled researchers to gain granular insight into the specific contributions made by discrete nucleoporins in the nuclear basket of the NPC to the export of mRNA. Specifically, by expanding upon the docking step facilitated by the protein TPR in the nuclear basket as well as identifying NUP153 as being the primary nuclear basket protein initiating export through the central channel of the NPC.

Figure 1:. A simplified diagram of nuclear pore complex regions and nuclear basket protein locations.

(A) A diagram of the nuclear pore complex broken into three distinct physical regions designated by red boxes. (B) A diagram of the nuclear pore complex highlighting the position of nuclear basket proteins with TPR (blue), NUP153 (orange), and NUP50 (green).

Figure 2:. Pathways of Nuclear Transport.

A simplified diagram illustrating the transport of cargo through the nuclear pore complex. As shown here, transport is bi-directional with transport receptors functioning to either import or export cargo, depending upon the transport receptor involved. The transport receptor associates with cargo and then transits through the NPC where it then releases its cargo. Upon release of cargo, the transport receptor then moves back through the NPC to begin the process anew.

Figure 3:. Three-Dimensional export routes of mRNPs.

(A-D) An axial view of the 3D probability density maps of mRNPs under different conditions. Shown in these panels are the entirety of the NPC, including the nuclear basket, central channel, and cytoplasmic fibrils. Red indicates high probability of localization while yellow indicates low probability of localization. (E-H) A radial view of the 3D probability density maps of mRNPs under different conditions. Here the radial view is broken into the discrete subsections of nuclear basket, central channel, and cytoplasmic fibrils. Partial figure reprinted with permission. Originally published in Proceedings of the National Academy of Sciences^[35].

Figure 4:. Simplified model of mRNA export.

(A) Formation: Following transcription and recruitment of co-factors, a transport receptor is recruited to form a competent mRNP. (B) Docking: The mRNP translocates to the nuclear basket where co-factors associate with TPR to facilitate docking. (C) Initiation: Initiation of export via interactions between NUP153 and the nuclear transport receptor. (D) Transport: Movement through the central channel facilitated by interactions between the transport receptor and FG-Nups. (E) Release: Release from the cytoplasmic fibrils. (F) Diffusion: The transport receptor disassociates from the mRNA and co-factors and allows the mRNA to translocate to the ribosome.