mRNA: Intranuclear Transport

Abstract

Most genes are transcribed in the nuclear interior and the messenger ribonucleic acid (mRNA) must then move from its transcription site to the nuclear pores to be transported to the cytoplasm for translation. mRNAs interact with multiple proteins both during and after transcription, some of which help to determine their export competency. High‐resolution, live cell tracking studies show that when released from the transcription site, messenger ribonucleoprotein (mRNP) complexes move out into the nucleoplasm in all directions by simple diffusion. The mRNPs sometimes move discontinuously depending on obstacles they encounter and may be corralled in certain regions for a time, but overall, the nascent mRNPs rapidly distribute throughout the entire channel‐like interchromatin space. Export‐competent mRNPs then randomly encounter a nuclear pore and are captured for export to the cytoplasm.

Key Concepts:

  • mRNPs released from the transcription site move through the nucleoplasm primarily by diffusion.

  • Diffusing mRNPs can be corralled by encounters with obstacles such as chromatin.

  • Export‐competent mRNPs randomly encounter nuclear pores and are transported to the cytoplasm.

  • A minority of mRNAs are transcribed at the nuclear periphery and it is possible that these exit the nucleus through the nearest nuclear pore.

Keywords: transcription; mRNA; mRNP; intranuclear movement; nucleocytoplasmic transport; nuclear structure; mRNA export; diffusion

Figure 1.

Intranuclear localisation of uncaged FL–oligo(dT) compared to chromatin distribution. Cells were incubated sequentially with caged FL–oligo(dT) and Hoechst 33342 and three‐dimensional stacks in both (a,c,e) blue (Hoechst‐labelled chromatin) and (b,d,f) green (uncaged FL–oligo(dT)) channels were captured and deconvolved (Politz et al., ). (a, b) Raw and (c,d) deconvolved midsections show the distribution of Hoechst signal and uncaged FL–oligo(dT) signal in the same nucleus. (e,f) The same images as in (c,d) but high‐intensity regions of Hoechst signal were (e) outlined and (f) the outlines superimposed on the oligo(dT) image. (g) A colour encoded overlay in which the Hoechst signal is green and the oligo(dT) signal is red. (h) A plot (linescan) of the intensity (arbitrary units) versus pixel number for the Hoechst (green) and oligo(dT) (red) signals as they vary along a line across the middle of (g). For (a–g), each image is approximately 19×19 μm. Reproduced from Politz et al. , with permission from Elsevier.

Figure 2.

(a) First frame of a movie (green, mRNPs; red, red fluorescent protein (RFP)–tubulin). (b) Maximum projection (inverted). (c) Enlarged area (from red box in (b)) showing nucleoplasmic tracks (red arrows) and nucleoli (Nu). (d) Maximum projection of mRNPs imaged for two different periods in the same cell; red shows tracks from a 20‐s movie; for 1 min later, green shows tracks from another 20‐s movie. White spots represent one of the nuclear tracks. (e) Hoechst‐labelled chromatin (pseudo‐coloured red) and mRNPs (green) were imaged in three dimensions in a living cell. (f) Close‐up of a portion of the nucleus. (g,h) Three‐dimensional rendering showing the nuclear volumes and mRNPs between chromatin regions. (g) Slices through the centre of the nucleus are given, (h) gives a top‐down view. (i,j) Maximum time projection (i) showing that mRNPs (green; red, RFP–tubulin) do not travel through nucleoli (yellow arrows) seen in DIC (j). (k) Maximum time projection (imaged every 1 s for 80 s) of a 1/2‐Mini‐Dys+intron nucleus (inverted). (l) Enlargement of marked area in (k), showing a nucleoplasmic track used by several mRNPs. Bars, 10 μm. Reproduced from Mor et al. , with permission from Nature Publishing Group.

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Politz, Joan C Ritland, and Pederson, Thoru(Nov 2010) mRNA: Intranuclear Transport. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0005984.pub2]