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© The Rockefeller University Press, 0021-9525/2000//271 $5.00
The Journal of Cell Biology, Volume 148, Number 2, , 2000 271-282

Original Article

Electron Tomography Reveals Posttranscriptional Binding of Pre-Mrnps to Specific Fibers in the Nucleoplasm



Francesc Mirallesa, Lars-Göran Öfverstedtb, Nafiseh Sabria, Youssef Aissounic, Ulf Hellmand, Ulf Skoglundb, and Neus Visaa

a Department of Molecular Genome Research, Stockholm University, SE-106 91 Stockholm, Sweden
b Department of Cell and Molecular Biology, Karolinska Institute, SE-171 77 Stockholm, Sweden
c Institut Paoli Calmettes, INSERM-U119, Cancérologie Expérimentale, F-13009 Marseille, France
d Ludwig Institute for Cancer Research, SE-751 24 Uppsala, Sweden
Department of Molecular Genome Research, Stockholm University, SE-106 91 Sweden.46-8-16 64 8846-8-16 41 11

neus.visa{at}molgen.su.se

Using electron tomography, we have analyzed whether the Balbiani ring (BR) pre-mRNP particles in transit from the gene to the nuclear pore complex (NPC) are bound to any structure that could impair free diffusion through the nucleoplasm. We show that one-third of the BR particles are in contact with thin connecting fibers (CFs), which in some cases merge into large fibrogranular clusters. The CFs have a specific protein composition different from that of BR particles, as shown by immuno-EM. Moreover, we have identified hrp65 as one of the protein components of the CFs. The sequencing of hrp65 cDNA reveals similarities with hnRNP proteins and splicing factors. However, hrp65 is likely to have a different function because it does not bind to nascent pre-mRNA and is not part of the pre-mRNP itself. Taken together, our observations indicate that pre-mRNPs are not always freely diffusible in the nucleoplasm but interact with fibers of specific structure and composition, which implies that some of the posttranscriptional events that the pre-mRNPs undergo before reaching the NPC occur in a bound state.

Key Words: pre-mRNA • Balbiani ring particle • hrp65 • mRNA export • nuclear substructure

© 2000 The Rockefeller University Press

Abbreviations used in this paper: 3D, three-dimensional; BR, Balbiani ring; CF, connecting fiber; COMET, Constrained Maximum Entropy Tomography; ET, electron tomography; FGC, fibrogranular cluster; NPC, nuclear pore complex.


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