Transcript TABLE 12–1 Relative Volumes Occupied by the Major Intracellular

Intracellular Compartments
and Protein Sorting
Compartmentalization of Cells
Pages 695-712
Proteins Characterize Organelles
-They catalyze the reactions that occur in each organelle
-They selectively transport small molecules in and out of
its interior
-They serve as organelle-specific surface markers that
direct new deliveries of proteins and lipids to the
appropriate organelle
Major Intracellular Compartments
Organelle Function
Nucleus – Contains the genome and is the site for DNA and RNA
synthesis
Endoplasmic Reticulum – Produces most of the lipid for the rest
of the cell
-Functions in transport of proteins to the Golgi
-Functions as a store for calcium
Golgi Apparatus – Receives proteins and lipids from the ER and
dispatches them to several destinations
Mitochondria – Generates most of the ATP used by cells
Lysosomes – Degrades intracellular organelles and
macromolecules taken in from outside the cell
Endosomes – Contain material taken in from outside the cell
Peroxisomes – Contain enzymes involved in oxidative reactions
TABLE 12–1 Relative Volumes Occupied by the Major Intracellular
Compartments in a Liver Cell (Hepatocyte)
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
EM of a Liver Cell
-Organelles often have
characteristic positions in the
cytosol depending on
interactions with the
cytoskeleton.
-The ER and Golgi depend on
the microtubule array.
-Eucaryotic cells are 10-20
times larger linearly, but
1,000-10,000 times greater in
volume.
Specialization of Membrane Function
Development of Plastids
Evolution of Cell Nucleus and ER
Evolution of Mitochondria
Relationships between
Compartments
Protein Traffic
Map
Types of Protein Transport
1. Gated transport – Protein traffic between the nucleus and
cytosol occurs between topographically equivalent spaces,
which are connected through the nuclear pore complexes
2. Transmembrane transport – Membrane-bound protein
translocators directly transport specific proteins across a
membrane from the cytosol into a space that is topologically
distinct
3. Vesicular transport – Membrane-enclosed transport
intermediates ferry proteins from one compartment to another
Vesicle Transport
Types of Sorting Signals
Each type of protein transfer is usually guided by sorting signals
in the transported protein that are recognized by receptors. Most
receptors recognize classes of proteins rather than just one protein.
15-60 AA
Signal Sequences
Organelles Cannot be Constructed
from Scratch
During division, cells must duplicate their organelles
They do it by enlarging existing organelles by
incorporating new molecules into them and then
dividing
Each daughter cell inherits their organelles from their
mother
Nuclear Envelope
Defines the nuclear
compartment
Inner membrane contains
specific proteins that interact
with chromatin and the nuclear
lamina
Outer membrane is continuous
with the membrane on the ER
Nuclear Pore Complexes
-Composed of over 30 different proteins called nucleoporins
-The more active the nucleus is in transcription the more
complexes the envelope contains, typically there’s 3000-4000
Nuclear side
Nuclear Pore Complex
500 macromolecules per second
Free Diffusion through Nuclear Pores
9 nm diameter limit for free diffusion, but up to 39 nm
can be brought through by transport receptors
Function of
a NLS
NLS is rich in positively
charged amino acids,
lysine and arginine
Nuclear proteins can be
transported through a
pore complex while they
are in a fully folded
conformation
Visualizing Active Transport
Nuclear Import Receptors
The import receptors are soluble cytosolic proteins that bind both
the NLS on the protein to be transported and to nucleoporins.
Many of the nucleoporins contain phenylalanine-glycine (FG)repeats that serve as binding sites for the import receptors
Nuclear Export
Relies on nuclear export signals on proteins that are
bound by nuclear export receptors
Both types of receptors belong to the family of nuclear
transport receptors
In yeast there are 14 genes in this family, many more in
humans
A single pore complex conducts traffic in both directions
Import into nucleus -Pro-Pro-Lys-Lys-Lys-Arg-Lys-ValExport from nucleus -Leu-Ala-Leu-Lys-Leu-Ala-Gly-Leu-Asp-Ile-
Compartmentalization of Ran-GDP
and Ran-GTP
GAP – GTPaseactivating protein
GEF – Guanine
exchange factor
Ran is a GTPase
Directionality of Nuclear Transport
Ran Binding
Protein
Model for Cargo Release by Ran-GTP
Nuclear Transport in Drosophila
-Gene regulatory protein called
dorsal stained brown
-Expressed in the ventral nuclei
Control of Nuclear Import
during T-cell Activation
NF-AT – Nuclear
factor of activated
T cells
Regulation of
nuclear localization
is done by
phosphorylation
The Nuclear Lamina
Nuclear lamins - are
Intermediate filaments
-Gives shape and stability
to the nuclear envelope
-Interacts with chromatin
Assemble/Disassembly of the
Nuclear Lamina
-Lamin Phosphorylation
-NPCs disassemble,
disperse, bind nuclear
import receptors
-Motor proteins are
involved with
disassembly
-Nuclear envelope reassembles around chromosomes