El núcleo de las células

El núcleo de las células
Envoltura nuclear
Características del núcleo:
Una delgada membrana que permite la comunicación
directa entre el contido nuclear y el citoplasma a través
de los poros de está membrana.
Material líquido en el núcleo – nucleoplasma
El nucléolo
La red de cromatina formada por hebras de ADN
Funciones de núcleo:
Centro del control celular
Características hereditarias
Control de la división
Citoplasma
Figure 2-49. The structure of
the cytoplasm.
The drawing is approximately
to scale and emphasizes the
crowding in the cytoplasm.
Only the macromolecules are
shown: RNAs are shown in
blue, ribosomes in green, and
proteins in red. Enzymes and
other macromolecules diffuse
relatively slowly in the
cytoplasm, in part because they
interact with many other
macromolecules; small
molecules, by contrast, diffuse
nearly as rapidly as they do in
water. (Adapted from D.S.
Goodsell, Trends Biochem. Sci.
16:203-206, 1991.)
Compartimientos intracelulares.
Organelas
• A diferencias de las bacterias, que consisten en un sólo
compartimiento rodeado de una membrana celular, las
células eucariotas están elaboradamente subdivididas en
compartimientos rodeados de membrana funcionalmente
diferentes. Cada compartimiento o organela contiene su
propio grupo característico de enzimas y otras moléculas
especializadas, y sistemas complejos de distribución
transportan los productos específicos de un
compartimiento a otro.
• Para entender una célula eucariótica, es esencial saber
qué ocurre en cada uno de estos compartimientos, cómo
las moléculas se mueven entre ellos y cómo estos
compartimientos son creados y mantenidos.
Todas las células eucarióticas tienen el mismo grupo de organelas
rodeadas de membranas
Muchos procesos vitales tiene lugar en o sobre la superficie de las
membranas, por ejemplo el metabolismo de los lípidos es catalizado por
enzimas unidas a las membranas, la fosforilación oxidativa y la
fotosíntesis requieren de una membrana para transportar H+ y sintetizar
ATP.
El sistema de membranas intracelular, tiene más funciones que las de
incrementar la superficie:
• crean compartimiento cerrados separados del citoplasma, proveyendo
espacios acuosos especializados funcionalmente.
• debido a que las membranas son impermeables a la mayoría de las
moléculas hidrosolubles, cada membrana de cada organela debe contener
proteínas de transporte que son las responsables de importar y exportar
productos (metabolitos) específicos. Cada membrana de organela tiene un
mecanismo para importación e incorporación a la organela única.
Compartimientos intracelulares de una célula animal
Figure 12-1. The
major
intracellular
compartments of
an animal cell.
The cytosol (gray),
endoplasmic
reticulum, Golgi
apparatus,
nucleus,
mitochondrion,
endosome,
lysosome, and
peroxisome are
distinct
compartments
isolated from the
rest of the cell by
at least one
selectively
permeable
membrane.
•Retículo endoplásmico rugoso; ca 50% de las
endomembranas.
•Sitema de Golgi
•Mitocondrias y cloroplastos
•Otros plastídeos; almacenamiento de
alimento y pigmentos en células animales.
•Lisosomas
•Peroxisomas
INTRACELLULAR
COMPARTMENT
Cytosol
Mitochondria
Rough ER cisternae
Smooth ER
cisternae plus Golgi
cisternae
Nucleus
Peroxisomes
Lysosomes
Endosomes
PERCENTAGE OF
TOTAL CELL
VOLUME
54
22
9
6
6
1
1
1
Table 12-1.
Relative Volumes
Occupied by the
Major
Intracellular
Compartments in
a Liver Cell
(Hepatocyte)
Figure 12-2. An
electron
micrograph of
part of a liver
cell seen in cross
section.
Examples of
most of the
major
intracellular
compartments
are indicated.
(Courtesy of
Daniel S. Friend.)
Figure 12-3. Development of plastids. (A) Proplastids are inherited with the cytoplasm of plant egg cells. As
immature plant cells differentiate, the proplastids develop according to the needs of the specialized cell: they
can become chloroplasts (in green leaf cells), storage plastids that accumulate starch (e.g., in potato tubers) or
oil and lipid droplets (e.g., in fatty seeds), or chromoplasts that harbor pigments (e.g., in flower petals). (B)
Development of the thylakoid. As chloroplasts develop, invaginated patches of specialized membrane from the
proplastid inner membrane pinch off to form thylakoid vesicles, which then develop into the mature thylakoid.
The thylakoid membrane forms a separate compartment, the thylakoid space, which is structurally and
functionally distinct from the rest of the chloroplast. Thylakoids can grow and divide autonomously as
chloroplasts proliferate.
Figure 12-4. Hypothetical schemes for the
evolutionary origins of some membraneenclosed organelles. The origins of
mitochondria, chloroplasts, ER, and the cell
nucleus can explain the topological
relationships of these intra-cellular
compartments in eucaryotic cells. (A) A
possible pathway for the evolution of the cell
nucleus and the ER. In some bacteria the
single DNA molecule is attached to an
invagination of the plasma membrane. Such
an invagination in a very ancient procaryotic
cell could have rearranged to form an
envelope around the DNA, while still allowing
the DNA access to the cell cytosol (as is
required for DNA to direct protein synthesis).
This envelope is presumed to have eventually
pinched off completely from the plasma
membrane, producing a nuclear compartment
surrounded by a double membrane. As
illustrated, the nuclear envelope is penetrated
by communicating channels called nuclear
pore complexes. Because it is surrounded by
two membranes that are in continuity where
they are penetrated by these pores, the
nuclear compartment is topologically
equivalent to the cytosol; in fact, during
mitosis the nuclear contents mix with the
cytosol. The lumen of the ER is continuous with
the space between the inner and outer nuclear
membranes and topologically equivalent to
the extracellular space. (B) Mitochondria (and
plastids) are thought to have originated when a
bacterium was engulfed by a larger preeucaryotic cell. They retain their autonomy.
This may explain why the lumens of these
organelles remain isolated from the
membrane traffic that interconnects the
lumens of many other intracellular
compartments.
Figure 12-5. Topological relationships between compartments of the secretory and
endocytic pathways in a eucaryotic cell. Topologically equivalent spaces are shown in
red. In principle, cycles of membrane budding and fusion permit the lumen of any of
these organelles to communicate with any other and with the cell exterior by means of
transport vesicles. Blue arrows indicate the extensive network of outbound and
inbound traffic routes, which we discuss in Chapter 13. Some organelles, most notably
mitochondria and (in plant cells) plastids do not take part in this communication and
are isolated from the traffic between organelles shown here.
Figure 12-6. A simplified "roadmap" of protein
traffic. Proteins can move from one
compartment to another by gated transport
(red), transmembrane transport (blue), or
vesicular transport (green). The signals that
direct a given protein's movement through the
system, and thereby determine its eventual
location in the cell, are contained in each
protein's amino acid sequence. The journey
begins with the synthesis of a protein on a
ribosome in the cytosol and terminates when
the final destination is reached. At each
intermediate station (boxes), a decision is
made as to whether the protein is to be
retained in that compartment or transported
further. In principle, a signal could be required
for either retention in or exit from a
compartment. We shall use this figure
repeatedly as a guide throughout this chapter
and the next, highlighting in color the
particular pathway being discussed.
Figure 12-7. Vesicle budding and fusion
during vesicular transport. Transport
vesicles bud from one compartment
(donor) and fuse with another (target)
compartment. In the process, soluble
components (red dots) are transferred
from lumen to lumen. Note that
membrane is also transferred, and that
the original orientation of both proteins
and lipids in the donor-compartment
membrane is preserved in the targetcompartment membrane. Thus,
membrane proteins retain their
asymmetric orientation, with the same
domains always facing the cytosol.
Figure 12-8. Two ways in which a sorting signal can be built into a protein. (A) The
signal resides in a single discrete stretch of amino acid sequence, called a signal
sequence, that is exposed in the folded protein. Signal sequences often occur at the
end of the polypeptide chain (as shown), but they can also be located internally. (B)
A signal patch can be formed by the juxtaposition of amino acids from regions that
are physically separated before the protein folds (as shown). Alternatively, separate
patches on the surface of the folded protein that are spaced a fixed distance apart
can form the signal.
Tráfico núcleo – citoplasma - núcleo
Tráfico núcleo – citoplasma - núcleo
Figure 12-11. Possible paths for free diffusion through the nuclear pore complex. This drawing
shows a hypothetical diaphragm (gray) inserted into the pore to restrict the size of the open
channel to 9 nm, the pore size estimated from diffusion measurements. Nine nanometers is a
much smaller diameter than that of the central opening apparent on the images of the nuclear
pore complex derived from electron micrographs. It is also smaller than the opening estimated
during active transport, when the pore dilates to allow the transport of particles of up to 26 nm in
diameter (arrow). Thus, it is likely that some pore components are lost during the preparation of
specimens for electron microscopy, and that these normally restrict free diffusion through the
central opening. Such components may form a diaphragm (or plug) that opens and closes to allow
the passage of large objects during active transport, which depends on sorting signals (discussed
below). Although plugs can be seen in some preparations, it is not clear whether they are
components of the pore complex or material that is being transported through it. Threedimensional computer reconstructions suggest that the channels permitting free diffusion might
be located near the rim of the pore complex, between the column subunits, rather than at its
center (see Figure 12-10A); this would mean that passive diffusion and active transport take place
through different parts of the complex.
Figure 12-13. Visualizing active
import through nuclear pores. This
series of electron micrographs shows
colloidal gold spheres (arrowheads)
coated with peptides containing
nuclear localization signals entering
the nucleus by means of nuclear
pore complexes. Gold particles were
injected into living cells, which then
were fixed and prepared for electron
microscopy at various times after
injection. At early time points (10
min), gold particles are seen in
proximity to the cytosolic fibrils of
the nuclear pore complexes. They
then migrate to the center of the
nuclear pore complexes, where they
are first seen exclusively on the
cytosolic face (30 and 40 min) and
then appear on the nuclear face (50
min). These gold particles are much
larger in diameter than the diffusion
channel in the pore complex, which
implies that the pores have been
induced to widen to permit their
passage. (From N. Panté and U. Aebi,
Science 273:1729
Figure 12-14. Nuclear import receptors. (A) Many nuclear import receptors bind
both to nucleoporins and to a nuclear localization signal on the cargo proteins
they transport. Cargo proteins 1, 2, and 3 in this example contain different
nuclear localization signals, which causes each to bind to a different nuclear
import receptor. (B) Cargo protein 4 shown here requires an adaptor protein to
bind to its nuclear import receptor. The adaptors are structurally related to
nuclear import receptors and recognize nuclear localization signals on cargo
proteins. They also contain a nuclear localization signal that binds them to an
import receptor.
Figure 12-15. The compartmentalization of Ran-GDP and Ran-GTP. Localization of RanGDP to the cytosol and Ran-GTP to the nucleus results from the localization of two Ran
regulatory proteins: Ran GTPase-activating protein (Ran-GAP) is located in the cytosol
and Ran guanine nucleotide exchange factor (Ran-GEF) is bound to chromatin and is
hence exclusively found in the nucleus. Another protein, called Ran Binding Protein
(omitted here for clarity), collaborates with Ran-GAP in activating GTP hydrolysis.
Figure 12-16. A model for how GTP hydrolysis by Ran provides directionality for nuclear
transport. Movement through the pore complex of loaded nuclear transport receptors
may occur by guided diffusion along the FG-repeats displayed by nucleoporins. The
differential localization of Ran-GTP in the nucleus and Ran-GDP in the cytosol provides
directionality (red arrows) to both nuclear import (left) and nuclear export (right). The
hydrolysis of GTP to produce Ran-GDP is mediated by Ran-GAP and Ran Binding Protein
on the cytosolic side of the nuclear pore complex.
Figure 12-17. A model for how the binding of Ran-GTP might cause nuclear import receptors to
release their cargo. (A) Nuclear transport receptors are composed of repeated a-helical motifs
that stack into either large arches or snail-shaped coils, depending on the particular receptor or
adaptor. Cargo proteins and Ran-GTP bind to different regions at the inside faces of the arches.
In a co-crystal of a nuclear import receptor bound to Ran-GTP, a conserved loop (red) of the
receptor becomes covered by bound Ran-GTP, which, in the Ran-free state of the receptor, is
thought to be important for signal sequence binding. (B) The cycle of loading in the cytosol and
unloading in the nucleus of a nuclear import receptor. (A, adapted from Y. M. Chook and G.
Blobel, Nature 399:230
Figure 12-19. The control of nuclear import
during T-cell activation. The nuclear factor of
activated T cells (NF-AT) is a gene regulatory
protein that, in the resting T cell, is found in
the cytosol in a phosphorylated state. When T
cells are activated, the intracellular Ca2+
concentration increases. In high Ca2+, the
protein phosphatase, calcineurin, binds to NFAT. Binding of calcineurin dephosphorylates
NF-AT, exposing one or more nuclear import
signals, and it may also block a nuclear
export signal. The complex of NF-AT bound to
calcineurin is then imported into the nucleus,
where NF-AT activates the transcription of
numerous cytokine and cell-surface protein
genes that are required for a proper immune
response. During the shut-off of the response,
decreased Ca2+ levels lead to the release of
calcineurin. Rephosphorylation of NF-AT
inactivates the nuclear import signal, and it
re-exposes the nuclear export signal of NF-AT
causing NF-AT to relocate to the cytosol.
Some of the most potent
immunosuppressive drugs, such as
cyclosporin A and FK506, inhibit the ability of
calcineurin to dephosphorylate NF-AT; these
drugs thereby block the nuclear
accumulation of NF-AT.
Figure 12-21. The
breakdown and reformation of the
nuclear envelope
during mitosis. The
phosphorylation of
the lamins is thought
to trigger the
disassembly of the
nuclear lamina,
which in turn causes
the nuclear envelope
to break up.
Dephosphorylation of
the lamins is thought
to help to reverse the
process.
Tráfico de moléculas en peroxisomas
Electron micrograph of three peroxisomes in a rat liver cell. The paracrystalline
electron-dense inclusions are composed of the enzyme urate oxidase. (Courtesy of
Daniel S. Friend.)
Los Peroxisomas contienen una o más enzimas que utilizan O2
molecular para remover átomos de hidrógeno de sustratos orgánicos
específicos (R-) en una reacción oxidativa que produce H2O2
La Catalasa utiliza el H2O2 generada por otras enzimas en la
organela para oxidar una variedad de sustratos, incluyendo
fenoles, ácido fórmico, formaldehído y alcohol etílico a través de la
reacción peroxidativa: H2O2 + R’H2
R’ + 2H2O.
Esta reacción es relevante en el hígado, ya que detoxifican diversos
tóxicos que circulan por los vasos sanguíneos (por ejemplo, 25%
del alcohol etílico)
R
Figure 12-35. Fluorescent micrographs of the endoplasmic
reticulum. (A) Part of the ER network in a cultured
mammalian cell, stained with an antibody that binds to a
protein retained in the ER. The ER extends as a network
throughout the entire cytosol, so that all regions of the
cytosol are close to some portion of the ER membrane. (B)
Part of an ER network in a living plant cell that was
genetically engineered to express a fluorescent protein in
the ER. (A, courtesy of Hugh Pelham; B, courtesy of Petra
Boevink and Chris Hawes.)
Figure 12-36. The rough ER. (A) An electron micrograph of the rough ER in a pancreatic exocrine cell that
makes and secretes large amounts of digestive enzymes every day. The cytosol is filled with closely packed
sheets of ER membrane studded with ribosomes. At the top left is a portion of the nucleus and its nuclear
envelope; note that the outer nuclear membrane, which is continuous with the ER, is also studded with
ribosomes. (B) A thin section electron micrograph of polyribosomes attached to the ER membrane. The plane
of section in some places cuts through the ER roughly parallel to the membrane, giving a face-on view of the
rosettelike pattern of the polyribosomes. (A, courtesy of Lelio Orci; B, courtesy of George Palade.)
Figure 12-37. Free and
membrane-bound ribosomes. A
common pool of ribosomes is
used to synthesize the proteins
that stay in the cytosol and
those that are transported into
the ER. The ER signal sequence
on a newly formed polypeptide
chain directs the engaged
ribosome to the ER membrane.
The mRNA molecule remains
permanently bound to the ER
as part of a polyribosome, while
the ribosomes that move along
it are recycled; at the end of
each round of protein synthesis,
the ribosomal subunits are
released and rejoin the
common pool in the cytosol.
Figure 12-38. The smooth ER. (A) Abundant smooth ER in a steroid-hormonesecreting cell. This electron micrograph is of a testosterone-secreting Leydig cell in
the human testis. (B) A three-dimensional reconstruction of a region of smooth ER
and rough ER in a liver cell. The rough ER forms oriented stacks of flattened
cisternae, each having a lumenal space 20 - 30 nm wide. The smooth ER membrane
is connected to these cisternae and forms a fine network of tubules 30 - 60 nm in
diameter. (A, courtesy of Daniel S. Friend; B, after R.V. Krstic´, Ultrastructure of the
Mammalian Cell. New York: Springer-Verlag, 1979.)