lecture 13 Flashcards
describe ENaC
it has 3 subunits (alpha, beta, gamma) and 3 large extracellular loops with a C and an N terminus
each subunit has 2 transmembrane domains
what are some reasons that cause alterations in protein trafficing pathways
what are some diseases that happen when things go wrong
Genetic polymorphisms or mutations alter protein trafficking pathways to reduce or increase cell surface populations disrupting ion transport
pathways e.g. some examples of this that lead to diseases are….
Very little DF508-CFTR reaches the apical membrane causing cystic fibrosis. this happens because the protein is folded incorrectly leading to degradation within the cell
In Liddle’s syndrome ENaC endocytosis is inhibited causing severe hypertension (this mutation in on intracellular parts).
what causes cystic fibrosis
Very little DF508-CFTR reaches the apical membrane causing cystic fibrosis. this happens because the protein is folded incorrectly leading to degradation within the cell
what is Liddle’s syndrome
In Liddle’s syndrome ENaC endocytosis is inhibited causing severe hypertension (this mutation in on intracellular parts).
what is the general pathway of protein trafficking
Ribosomes translate mRNA into proteins
It then gets packaged and sent to the Golgi where it is modified and packaged into vesicles and sent to where the are supposed to be
Trans Golgi network is what tells the vesicles where to go
when would proteins have a hydrophobic signal sequence
when they are destined for a secretory pathway
eg they are destined for the ER, Golgi, cell surface
where would the hydrophobic signal sequence be located in secretory pathway proteins
at the N terminus or further into the protein
where do ENaC, CFTR and Na/K ATPase have there signal sequence
they have internal signal sequences in their transmembrane domains
ER locolised protein have what amino acid sequence
KDEL
(K=lysine, D=aspartic acid, E=glutamic acid, L=leucine).
KDEL tels the golgi to put the proteins back into the ER
what acts as an address as to where the proteins are supposed to go
the amino acid sequence
which is the first part of the protein to be translated
the N terminus
the hydrophobic region is usually in close proximity to this
describe how protein synthesis works for a simple protein (no transmembrane domains)
The ribosome locks onto some mRNA and protein synthesis will begin. At this point they are not in the Endoplasmic reticulum
There is a series of 10 - 15 hydrophobic amino acids which is the signal sequence. This will be recognised by a signal recognition particle which is floating around looking for the sequence
When it finds it, it will pull the mRNA along with the ribosome and the few amino acids that have been translated to the ER. When it brings it there, it will bind to the signal recognition particle receptor (SRPR) which is located very close to a translocon (Brown thing) which is located very close to a pore
When it binds the sequence it inhibits protein synthesis because it is bound to the hydrophobic region and in in the cytoplasm therefore it is not happy. therefore it has to be inside the ER before synthesis will resume (attached to the membrane receptor translocon).
the hydrophobic sequence (signal sequence) is cleaved by an enzyme called signal peptidase. Once the hydrophobic region is chopped off protein synthesis can be completed
At this point (after synthesis is compleated) the ribosome is able to disassociate away and the protein is inside the ER.
There is also an addition of carbs as in the ER sugars can be added onto proteins as the ER is where posttranslational modification begins
does the ribosome bind inside or outside of the ER
outside
what happens initially when the ribosome binds
it translates a series of 10-15 hydrophobic amino acids which is the signal sequence
what pulls the mRNA with the ribosome and the few amino acids that have been translated to the ER
the signal recognition particle
what receptor does the signal recognition particle bind
the signal recognition particle receptor
what is in close proximity to the signal recognition particle receptor
a translocon (pore)
what happens when the hydrophobic region is inside the ER attached to the receptor
signal peptidase cleaves off the signal sequence
allowing for the rest of the sequences to be translated
what sets up the orientation (topology) of membrane proteins
hydrophobic and positive stretches of amino acids
what is the difference in synthesis between a simple protein and a protein with 1 transmembrane domain
after signal peptidase cleaves of the signal sequence and translation starts again, it will come across another hydrophobic region that will want to stay in the membrane
This is called a stop transfer sequence. The stop transfer sequence will stay in the translocon and the rest of the protein will be made outside of the ER
When synthesis is compleart the ribosome will dissocosite. Outside the ER is the C terminal and inside in the N
Things made in the ER will end up outside the cell, cytosol part will end up inside the cell and the transmembrane is in the transmembrane
