protein sorting and transport Flashcards
what are proteins translated from?
mRNA
what are some functions of proteins?
receptor construction
supply components to the mitochondria
produces secretory proteins
produce components pf the cytosol
what are the functions of lysosomes?
protein degradation
phagocytosis and autophagy
what are the functions of the golgi complex?
protein processing and sorting
lipid and polysaccharide synthesis
what are the functions of the rough endoplasmic reticulum?
protein targeting
protein folding and processing
quality control
what is gated transport?
movement of proteins between the cytoplasm and nucleus
what is transmembrane transport?
movement of proteins across a membrane from the cytoplasm
what is vesicle transport?
building a vesicle from one membrane to another
what were Palades discoveries?
used pancreatic cells and conducted the ‘pulse chase’ experiment
found the first membrane organelle that the protein enters
protein enters the rough endoplasmic reticulum then vesicles
also discovered that newly synthesised proteins are labelled
what were Blobel and Sabatini’s discoveries?
discovered signal sequences
if the proteins are destined to be secreted into the cytoplasm then the protein has longer sequences
what are signal sequences?
a stretch of around 20 hydrophobic amino acids usually located at the amino terminus of the polypeptide
it tells the cell where the protein is destined for
what is co-translational translocation?
when proteins are targeted to the ER during translation
what is post-translational translocation?
when proteins are targeted to the ER after translation
explain co-translational targeting of secretory protein into the ER
translating proteins emerge from the ribosome and contains a signal sequence
signal recognition particle engages with the sequence bringing it to the signal recognition particle on the ER and is then released
translocon on the ER membrane opens
signal peptidases cleave the recognition sequence and translation continues
explain post-translational translocation of proteins into the ER.
proteins are synthesised on free ribosomes
signal sequence is recognised by receptor proteins (Sec 62/63 and Sec 70/71) that are associated with the translocon in the ER membrane
cytosolic heat shock protein 70 chaperones maintain the polypeptide in an unfolded conformation in the cytosol so they can enter the translocon
BiP pulls the polypeptide through the channel
are proteins destined for secretion translocated into the ER lumen or inserted into the ER membrane?
translocated into the lumen
are proteins destined for the plasma membrane translocated into the ER or inserted into the ER membrane?
inserted into the ER membrane
explain insertion of membrane proteins with internal transmembrane sequences.
signal recognition particle receptor binds to signal sequence and opens the translocon
translation occurs within the translocon and the protein is then inserted into the membrane with either is C or N terminus exposed
what are examples of post translational modifications?
phosphorylation
glycosylation
which Hsp70 chaperone ensures correct protein folding in the ER?
BiP
it binds to the polypeptide chain as it crosses the ER membrane
what proportion of proteins are misfolded in the ER?
half
which glycoprotein chaperones are sensors of misfolded proteins?
calnexin
calreticulin
what protein recognises severely misfolded glycoproteins?
EDEM1
what does EDEM1 remove?
mannose residues from glycoproteins
what happens to severely misfolded proteins in the ER?
EDEM1 recognises the misfolded proteins and removes mannose residues
protein is then returned to the cytosol through a ubiquitin ligase complex which labels the protein with a ubiquitin molecule which is recognised by a proteasome
it is then degraded
what does a mutation in alpha 1-antitrypsin lead to?
misfolding of the protein and retention in the ER
what are the normal functions of alpha 1-antitrypsin?
protease inhibitor
involved in various immunoregulatory functions
what are the effects of a mutation in alpha 1-antitrypsin ?
lung tissue damage as it usually protects lungs from proteases so degradation of the ECM leads to COPD
build up can lead to liver cirrhosis
what does the abbreviation ‘ERGIC’ stand for?
ER-golgi intermediate complex
explain vesicular transport from the ER to the golgi
luminal proteins containing export signal bind to transmembrane proteins
they are then incorporated into a transport vesicle which buds from the ER
the vesicle then binds with the cis golgi membrane and the protein enters
what are the targeting sequences that cause proteins to be retained in the ER?
KDEL sequence: Lysine- Aspartic acid- Glutamic acid- Leucine
KKXX sequences
what happens if proteins which are supposed to be retained in the ER are accidentally transported out?
they are recognised by KDEL receptors and are transported back to the ER
what are the different regions of the golgi apparatus?
cis
medial
trans
what are synthesised in the Golgi apparatus?
glycolipids and sphingomyelin
what does the cisternae maturation model propose?
that secretory cargo travel in cisternal compartments that slowly mature from the cis-Golgi to the trans-Golgi
says that the cisternae move and mature through accumulating trans then medial enzymes
what does the stable cisternae model propose?
that the cisternae remains in one place with unchanging enzymes
secretory cargo travel from one golgi compartment to the next in COP I vesicles
exit is clathrin coated vesicles from the trans golgi
what is the constitutive secretory pathway?
continuous secretion of proteins from the cell
what is regulated secretion?
specific proteins are secreted in response to environmental signals
what regulates the formation of coated vesicles?
small GTP-binding proteins related to Ras and Ran
explain budding and fusion of a transport vesicle.
assembly of coat proteins drives the budding of vesicles containing selected cargo proteins from the donor membrane
the coats are removed at the target membrane allowing the membranes to fuse and the vesicles to be emptied
what are the 3 kinds of coated proteins?
COP I
COP II
clathrin coated
what is the function of COP I- coated vesicles?
transport ER proteins marked by KDEL or KXX retrieval signals from the ERGIC or the cis golgi network to the ER
what is the function of COP II coated vesicles?
bud from the ER and transports its cargo to the ERGIC or golgi apparatus
what is the function of clathrin coated vesicles?
responsible for the uptake of extracellular molecules from the plasma membrane by endocytosis and from trans golgi to endosomes, lysosomes or the plasma membrane
what regulates the assembly of COPI and clathrin vesicles?
ARF GTPase
what regulates the assembly of COPII vesicles?
Sar 1
explain the initiation of clathrin-coated vesicles by ARF1
ARF 1/ GDP binds to proteins in the golgi membrane
ARF- guanine nucleotide exchange factor in the membrane stimulates the exchange of GDP to GTP
ARF/GDP initiates budding by recruiting adapter proteins
these serve as binding sites for clathrin as well as transmembrane proteins
explain coat disassembly of clathrin coated vesicles
GTP bound to ARF1 is hydrolysed to GDP
ARF1/GDP is released from the membrane for recycling
loss of ARF1 and the action of uncoating enzymes weakens the binding of the clathrin coat complex
this allows chaperone proteins in cytoplasm to dissociate the coat
what are snare proteins?
membrane bound proteins that help a vesicle fuse with the cell membrane
how do vesicles fuse with the cell membrane?
Rab/GDP is converted to Rab/GTP by specific guanine exchange factors
Rab/GTP interacts with effector proteins and v-SNARES to assemble a pre-fusion complex on the transport vesicle
rab protein on target membrane organises effector proteins and t-SNARES
when the vesicle comes close to the target membrane the effector proteins link membranes
this stimulates Rab/GTP hydrolysis and allows v-SNARES to connect to t-SNARES
what is Grescilli’s syndrome?
a rare disease caused by mutation in the gene encoding Rab27a
what does Rab27a play a key role in transporting?
melanosomes
pigment-containing vesicles
exocytosis of vesicles in T lymphocytes
what are the symptoms of Griscilli’s syndrome?
partial albinism
immunodeficiency
what is the normal function of Rab27a?
functions as a motor adapter
connects the melanosomes to actin motor myosin
melanosomes are then transported to the plasma membrane by Rab27a, Slac2a and myosin 5a
which type(s) of Grescilli’s syndrome causes loss of pigmentation?
1 and 3
which type(s) of Grescilli’s syndrome causes neurological dysfunction?
1
which type(s) of Grescilli’s syndrome causes immunological defects?
2
what causes loss of pigmentation in Grescilli syndrome?
a mutation in Rab27a or myosin 5a causes disturbance to movement to actin rich region in the plasma membrane
ultimately stops the melanosome reaching the plasma membrane which prevents exocytosis
what causes neurological dysfunction in Grescilli syndrome?
mutation in Myo5a gene prevents the transport of vesicles to neuronal synapses
what causes immunological defects in Grescilli syndrome?
caused by mutations in either Rab27a or Munc13-4
Rab27a disturbs the targeting of secretory lysosomes of the cytotoxic T cells to the immunological synapse
Munc13-4 disturbs the priming of lysosomes for fusion with the plasma membrane
how many degenerative enzymes do lysosomes contain?
60
what causes Gaucher disease?
a deficiency of glucocerebrosidase
in the most common form only macrophages are affected
characterised by enlarged liver and spleen
what is the function of glucocerebrosidase?
catalyses the hydrolysis of gucosylceramide to glucose and ceramide
what is the effect of a missense mutation in the gene encoding for glucocerebrosidase?
leads to misfolding in the ER
prevents the breakdown of glucosylceramide which is toxic when it accumulates
causes the affected cell to swell
