Chemistry Reference
In-Depth Information
8.4
RNA Dynamics in the Cytoplasm
Once the transcripts reach the cytoplasm, they move from the pore to disperse in the
environment. We can divide them into two different classes of RNAs based on their
final distribution: non-localizing and localizing RNAs. The
first will uniformly
distribute in the cytoplasm while the latter will be con
ned or enriched in speci
c
areas. Nevertheless both have the ability to move. Studies with inert tracers suggest
that the cytosol is heterogeneous with viscoelastic behavior, allowing limited diffu-
sion for particles of sizes similar to RNPs [55
58]. Therefore, unlike the situation in
the nucleus, the particles may require active transport if diffusion is impaired or
inef
cient. However their dynamics should differ to allow the observed speci
c
compartmentalization. In particular since distances are much larger in the cyto-
plasm, for instance the distal region of a neuron, amechanism is required to facilitate
transport.
-
8.4.1
Non-localizing RNA
Most mRNAs, such as housekeeping mRNAs, appear to belong to the non-localizing
class although their distribution may in fact be non-homogeneous (for instance
mRNAs formitochondrial proteins appear to be nearmitochondria [59]). Their role is
to spread out in the cytosol to ensure that their protein products will be generally and
uniformly available. The dynamics of single and speci
c RNAs in living cells has been
observed and measured by exploiting the MCP system in COS cells [60].
Three reporter genes with the MBS inserted and different 3
0
UTR (3
0
UnTranslated
Region) sequences either from human growth hormone (hGH mRNA) gene, SV40
(SV mRNA) or
-actin (as a control for known localizing RNA) were used. The
rst
two reporters exhibited four possible movements (Figure 8.3): static (33
-
40%),
corralled (
b
40%), diffusional (15
-
25%) and directed (2
-
5%). Interestingly, individ-
ual particles were able to switch between these movements and no correlation was
observed between a speci
c behavior and a particular area in the cells. Since active
transport is usually associated with cytoskeleton components, this hypothesis was
investigated by treating cells with speci
c drugs against microtubules and micro-
filaments. The results con
rmed the crucial role played by the cytoskeleton in
anchoring static particles, supplying tracks for directed motion and creating restrict-
ed areas not accessible to the particles, possibly transforming their diffusion into
corralled motion. The new
nding that non-localizing RNPs are also subjected to
directed movements suggests the involvement of active transport by molecular
motors onmicrotubules similar to localized RNPs. Actually, both RNP classes moved
with the same average speed (1
m/s) but the localized RNP classes used active
motion more frequently and for longer distances. Therefore, molecules switch
stochastically between various movements, but each RNA will have a speci
c
probability of displaying each of the four movements dependent on its sequence.
If a sequence enables the recruitment of factors interacting directly with the motors
1.5
m
-
