Chp. 15 Flashcards
Contains many metabolic pathways, protein synthesis, and cytoskeleton
Cytosol
Synthesis of most lipids, synthesis of proteins for distribution to many organelles and to plasma membrane
ER
Modification, sorting, packing of proteins and transport
Golgi
Intracellular degradation
Lysosomes
Sorting of endocytosed material
Endosomes
oxidative breakdown of toxic molecules
Peroxisomes
Membr ane-enclosed organelles occupy nearly __ of the cell volume
50%
Membrane-bound organelles allow __ of different cellular functions
spatial separation
With the exception of the nucleus, these organelles communicate extensively with one another and with the outside of the cell via
vesicular traffic
Nuclear envelope and the membranes of the ER, Golgi apparatus, endosomes and lysosomes likely originated by
invagination of the
plasma membrane
Mitochondria and chloroplasts likely evolved from
bacteria that
were engulfed by primitive eukaryotic cells
Mitochondria and chloroplasts remain…
isolated from the vesicular traffic
Protein transport begins with a
Signal sequence
The synthesis of virtually all proteins begins in the
Cytosol
The fate of each protein depends on whether it contains a
signal sequence, which directs the protein to a particular
organelle
Proteins that lack a signal sequence remain in the
Cytosol
The mechanism by which a protein is transported into an
organelle depends on the
Organelle
Transport of folded proteins into
Nucleus through nuclear pore
Transport of unfolded
proteins into the
ER, mitochondria or chloroplasts across their membranes by
protein translocators
Transport of proteins in
transport vesicles that
pinch
off from the ER and fuse
with a compartment of the
endomembrane system
On the cargo protein is it sequence specific signal
Yes
Signal sequence is recognized by
receptor proteins
Directional movement by transport machinery requires
Energy
Large pore, competent for fully folded proteins/complexes. GTP hydrolysis provides energy
nuclear import
narrow translocation channel;
proteins are unfolded and pulled into organelles by chaperone proteins (which hydrolyze ATP for energy)
Mitochondria/chloroplasts
Proteins enter as they are being synthesized in the
ER
__ are more important than the exact sequence
Hydrophobicity or the order of charged amino acids
Deleting a signal sequence from an ER protein converts it into a
cytosolic protein
Adding an ER signal sequence to a cytosolic protein directs it to the
ER
The outer nuclear membrane is continuous with the
ER
What part of the nucleus contains proteins that act as
binding sites for chromosomes and the nuclear lamina
Inner nuclear membrane
Transport across the nuclear
envelope occurs through
Nuclear pores
Nuclear pore complex acts as a
Selective gate
Many of the proteins that line this are unstructured and form a mesh that fills the pore, preventing passage of large molecules
Nuclear pore complex
Large structure composed of ~30 different proteins
Nuclear pore complex
Proteins enter the nucleus in what state
Mature, fully folded
Signal sequence that directs transport
into the nucleus is called a
Nuclear localization signal
NLS is recognized and bound by a cytosolic protein called a
Nuclear import receptor
facilitates nuclear transport
GTP hydrolysis
present in high concentrations in the nucleus
Ran-GTP
present in high concentrations in the cytosol
Ran-GDP
small monomeric GTPase
Ran
In the __, Ran GTP binds a
Nucleus, Nuclear import receptor
The import receptor, bound to __, returns to the __
Ran-GTP, cytosol
Hydrolysis of GTP causes…
Ran-GDP to release the import receptor
Proteins must unfold to enter
Mitochondria or chloroplasts
help pull in and refold proteins (use energy from
ATP hydrolysis)
Mitochondrial chaperone proteins
Simultaneous transport across the outer and inner membranes by __. The protein is __ during the transfer process
Protein translocaters, unfolded
Two types of proteins are transferred to the ER
Soluble, transmembrane
are translocated across the membrane into the ER lumen
Soluble proteins
are partly translocated across the membrane and
remain embedded in it
Trans membrane proteins
Proteins enter the ER while being synthesized and are directed there
by a
Hydrophobic ER signal sequence
A protein will not re enter the cytosol once inside the __. Instead, proteins are transported to their destination in __
ER lumen, or embedded in ER membrane; transport vesicles
Mos t proteins are threaded across the __ membrane before they are completely synthesized
ER
Membrane-bound ribosomes
create the
Rough ER
The ribosome synthesize the protein attached to the
ER membrane to initiate transfer
The same pool of ribosomes synthesizes
cytosolic proteins and ER proteins
Membrane-bound and free
(cytosolic) ribosomes are
Structurally identicical
When a ribosome is
synthesizing a protein with an
ER signal sequence, the signal
sequence directs the ribosome
To the
ER
Many ribosomes can bind the
same __ molecule, forming
a __
MRNA, polyribosome
binds both the ribosome and the ER signal sequence as it emerges from the ribosome
Signal recognition particle
Protein synthesis slows down until the SRP binds an __ in
the ER membrane
SRP receptor
Once SRP binds to an SRP receptor, the SRP is __ and passed to a __
Released, protein translocater
Soluble proteins made in the ER are released into the
ER lumen
The ER signal sequence opens the __ and remains bound during synthesis
P rotein translocator
initiates transfer into the ER
N-terminal ER signal sequence
located further along the polypeptide chain, halts the
transfer
stop-transfer sequence
Once the N terminal signal sequence is cleaved, the __ remains in the belayer, forming an __
Stop transfer sequence, alpha helix
The protein has a defined and permanent orientation. What is it?
N -terminus in the ER lumen,
and C-terminus in the cytosol
In some proteins, an internal signal sequence called a __ initiates protein transfer into the ER
start-transfer sequence
Start-transfer sequences and stop-transfer sequences can work in
Pairs to thread multi pass transmembrane proteins into ER
Are signal sequences removed from the protein once it’s been sorted?
Often, but not always
In the mitochondria, protein synthesis and folding occurs where
Cytosol
Do the following phrases describe the import of proteins in the nucleus, mitochondria or both?
The imported protein encodes a nuclear localization signal
Nucleus
The imported protein is pulled into the organelle’s lumen by chaperone proteins in…
Mitochondria
Protein is first synthesized and folded in the cytosol before
being imported in….
Nucleus and mitochondria
Important of proteins requires ATP in
Mitochondria
Important of proteins requires GTP in
Nucleus
Proteins are imported in their folded state in the
Nucleus
Proteins are imported in their unfolded state in…
Mitochondria
Where will a protein with an ER signal sequence be transported
Into the lumen of the ER
Where will a protein with an ER signal sequence and a stop transfer sequence be transported
Into the membrane of ER then crosses the membrane 1 time
Where will a protein with a start transfer sequence and a stop transfer sequence be transported
Embedded in the ER membrane and crosses the membrane twice
Continual budding and fusion of ___ enables transport
from the ER to the Golgi apparatus, and from the Golgi to other
compartments
transport vesicles
Vesicular transport allows
exocytosis and endocytosis
What is the secretory pathway
ER to Golgi to cell surface
Endocytic pathway
Plasma membrane to endosomes to lysosomes
allows ingestion and degradation of extracellular molecules
Endocytosis
Vesicular transport is
highly selective
The formation of transport vesicles is energetically __
unfavorable
__ is maintained during transport (i.e., the
cytosolic surface remains cytosolic; the extracellular/lumenal
surface remains extracellular/lumenal)
membrane topology
Best-studied vesicles have a protein coat made of the protein
clathrin
Clathrin molecules assemble into a basketlike network on the cytosolic side of the membrane, forming a
clathrin-coated pit
Mechanism budding of a clathrin-coated vesicle
- Cargo receptors bind molecules selected for transport
- Adaptins capture cargo receptors and bind clathrin (forming coated pit)
- Dynamin assessmles as a ring and hydrolyzes GTP to help pinch off vesicles
- Following budding, the vesicle sheds its protein coat
Uncoating of clathrin requires energy from
ATP hydrolysis
Dynamin is a
GTPase
__ on the vesicle surface are recognized by tethering
proteins on the target membrane
Rab proteins (GTPases)
act as molecular markers for each membrane type
Rab proteins
What ensures that transport vesicles fuse only with the correct membrane
Matching Rab and tethering protein
what drives vesicle fusion
SNARE
Movement between compartments is mediated by
transport vesicles
Vesicle budding is driven by
protein coat assembly (clathrin, COP)
Vesicle Scission is driven by assembly and
GTPase activity of Dynamin
Fusion is driven by
v-SNARE/t-SNARE winding
Rabs/tethering proteins provide
selective vesicle docking
Most proteins are covalently modified in the
ER
Disulfide bond formation is catalyzed by an
enzyme in the ER lumen
help stabilize the structure of secreted proteins
Disulfide bonds
Many proteins are glycosylated in the
ER
Oligosaccharides linked to an asparagine side chain are are called
N-linked
Subsequent modification of the oligosaccharide begins in the
ER
A branched oligosaccharide containing 14 sugars is transferred from a lipid
(called dolichol) to the side chain of an
asparagine amino acid
helps protect a protein from degradation by
preventing binding of proteases
Glycosylation
can help mediate binding to chaperone proteins (ensuring quality control)
Glycosylation
can serve as a transport signal for packaging the protein into an appropriate transport vesicle
Oligosaccharides
Oligosaccharides on the cell surface can function in
cell-cell
recognition
hold proteins in the ER until proper folding or assembly occurs
Chaperones
If proper folding fails, the proteins are exported to the cytosol for degradation
in the
proteasome
Misfolded proteins can accumulate in the ER, triggering the
unfolded protein response (UPR)
If the ER cannot keep up with the demand, the UPR directs the cell to
apoptosis
Golgi apparatus is a collection of flattened, membrane-enclosed sacs
called
cisternae
Proteins and lipids travel in transport vesicles from the ER and fuse with
the
cis Golgi network
Proteins exit the golgi from the
trans golgi network
operates continually in all eukaryotic cells and
does not require a particular signal sequence
Constitutive exocytosis
operates only in cells that are specialized for
secretion (i.e., secretory cells)
Regulated exocytosis
common activator of regulated secretion
calcium
Proteins destined for regulated
secretion…
aggregate with each other, packed onto high concentrations
Secretory vesicles bud from the trans Golgi network and accumulate
near the
plasma membrane
Influx of Ca2+ stimulates from pre-synpatic cell results in
fusion of synaptic vesicles
Influx of Ca2+ stimulates from pre-synpatic cell results in
fusion of synaptic vesicles
Ingestion of fluid and molecules via small pinocytic vesicles; carried out continually by all eukaryotic cells
Pinocytosis
Ingestion of large particles; mainly carried out by phagocytic cells
Phagocytosis
Ingested materials are delivered to
endosomes
Material to be ingested is enclosed by the plasma membrane,
which buds inward to form an
endocytic vesicle
Pseudopods fuse at their tips to form a
phagosomes
phagosomes fuse to
lysosomes
Pinocytosis is carried out mainly by
clathrin-coated pits and vesicles
enables selection of macromolecules for ingestion via binding to complementary
receptors on the cell surface (e.g., transport of cholesterol/LDL)
Receptor-mediated endocytosis
Empty receptors return either to their
original plasma membrane domain
(___) or to a different domain of
the plasma membrane
(___). Other receptors travel
to the lysosome for __.
recycling, transcytosis, degradation
principal sites of intracellular digestion
lysosomes
Lysosomes contain ~40 types of
hydrolytic enzymes active in lumen
Enzymes and membrane proteins of the lysosome are synthesized in the
ER
A phosphorylated sugar group,___, is added to these proteins while they are in the cis Golgi network.
mannose 6-phosphate
This sugar is recognized by the mannose 6-phosphate receptor, enabling sorting
into transport vesicles for
delivery to lysosomes via endosomes
is used to degrade obsolete parts of the cell (like organelles) via the
formation of a double membrane around the material to be ingested
autophagy
Transmembrane protein that binds molecules selected for transport
Cargo receptor
Assembles as a ring around the neck of a clathrin-coated pit
Dynamin
GTPase that acts as a marker on the surface of transport Rab vesicles and is recognized by tethering proteins on the target membrane
Rab
Pairs of these proteins wind around one another, pulling the vesicle’s membrane close to the target membrane and promoting fusion
SNARE
protein that captures cargo receptors and secures the
clathrin coat to the vesicle membrane
Adaptin
Assembles into a basketlike network that gives budding
vesicles their shape
Clathrin
True or false
____Early endosomes mature into late endosomes as they fuse with each other or with pre-existing late endosomes.
_______The interior of an endosome is kept basic (pH 8-9) by an ATP- driven proton pump located in the endosomal membrane.
_______ Endocytic cargo that dissociates from its receptor in the endosome is destined for degradation in the lysosome.
_______ Proteins destined for the plasma membrane are tagged with mannose-6-phosphate.
_______ After pinching off from the plasma membrane, clathrin-coated vesicles shed their coat and fuse with the Golgi apparatus.
T
F
T
F
F
