Chemistry Reference
In-Depth Information
adaptor bridging the RNA/REF complex with the NPC component, namely
nucleoporins with FG repeats (see below).
8.3.3
Transport through the Nuclear Pore Complex
The dynamics of particle translocation through the nuclear pore complex (NPC) are
still unclear and the mechanism for transport is under investigation [46]. Three types
of transport are associated with the pore in the nuclear envelope (NE): restricted
diffusion, facilitated diffusion and unidirectional receptor-dependent transport. If
the molecule does not interact with the nucleoporins, protein constituents of the
NPC, it is de
ned as inert and permeates through the internal channel by restricted
diffusion with a rate inversely proportional to its molecular mass, with a limiting size
of 50 kDa. Particles interacting directly with the nucleoporin FG repeats, usually
transport receptors like NTF2 and transportin 1, are subjected to facilitated translo-
cation. Both these mechanisms are passive bidirectional processes while transport
mediated by the receptor is an active unidirectional transport that proceeds against
the concentration gradient of the cargo proteins. A cargo is an inert molecule that
cannot diffuse freely through the pore. Instead they harbor speci
c signals (Nuclear
Localization Signal, NLS and Nuclear Export Signal, NES) bound by the adaptor to be
translocated to the right compartment [47]. Also in this case the translocation process
per se is not an energy-consuming task, since is not directly coupled with ATP
hydrolysis. The real energy driving the transport mediated by importin and exportin
proteins is the chemical potential of the RanGTP gradient maintained by NTF2 and
RanGEF. This latter protein recharges the RanGDP imported into the nucleus by
NTF2 with GTP. The RanGTP gradient, higher in the nucleus than in the cytoplasm,
is important for the correct directionality of the cargo transport, since association of
the receptor with the cargo is in
uenced by its level. Importins load the cargo at low
levels of RanGTP in the cytoplasm while in the nucleus high RanGTP levels trigger
the replacement of the cargo with RanGTP. The exportins work in the opposite
direction and with an opposite mechanism: they load the cargo only in combination
3
Figure 8.2 Live-cell imaging and single-particle
tracking of individual mRNPs in the nucleus of a
mammalian cell. Images from time-lapse films
acquired from a cell co-transfected with (A) CFP-
Lac repressor that marks the insertion locus and
(B) YFP-MCP. (C) Reduction of noise for tracking
of mRNPs was obtained by deconvolution. Bar,
2
(I) Plot of the area per frame traveled throughout
the tracking period. Diffusive particles are shown
in blue, corralled in green, stationary in yellow,
and transcription site in red. (J) Mean-square
displacement (MSD) of tracked nucleoplasmic
particles versus time indicated the presence of
three types of characterized movements:
diffusive (black circles), corralled (blue
triangles), and stationary (green squares).
Directed movement was never detected (red
dotted line). (K) Table summarizing the mean
velocities and diffusion coefficients of tracked
particles at 37
C. (Adapted from [23]).
m. (D) Tracking of mRNP (arrow,
transcription site) (bar, 2
m
m) showed (E)
diffusing particles, (F) corralled particles, (G)
stationary particles, and (H) the transcription
site. Tracks are marked in green, and time in
seconds from the beginning of tracking for each
particle that appears in each frame. Bars, 1
m
m
m.
