F. Miralles et al., Electron tomography reveals posttranscriptional binding of pre-mRNPs to specific fibers in the nucleoplasm, J CELL BIOL, 148(2), 2000, pp. 271-282
Using electron tomography, we have analyzed whether the Balbiani ring (BR)
pre-mRNP particles in transit from the gene to the nuclear pore complex (NP
C) 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 fibrogr
anular 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 c
DNA reveals similarities with hnRNP proteins and splicing factors. However,
hrp65 is likely to have a different function because it does not bind to n
ascent 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 t
he nucleoplasm but interact with fibers of specific structure and compositi
on, which implies that some of the posttranscriptional events that the pre-
mRNPs undergo before reaching the NPC occur in a bound state.