Membrane Proteins Flashcards
what is the process of eukaryotic protein synthesis?
ribosomes bind to mRNA and direct synthesis of polypeptides
what does the newly formed polypeptide contain?
- information or a post code that directs the polypeptide to a particular site in the cell
what is a signal peptide?
- has a signal
- co-translational ER targeting: peptide targets ribosome and protein to the ER and is continued to be translated
- get the formation of membrane proteins and luminal/secretory proteins
what proteins are produced when there is no signal?
- forms a cytosolic protein
- can be targeted to organelles
what do the majority of type II membrane proteins have?
- an internal sequence
what is protein targeting?
- pieces of information that enable the cellular transport machinery to correctly position a protein
- information contained in the polypeptide chain or in the folded protein
- continuous stretch of amino acid residues in the chain that enables targeting called signalling or targeting peptides
what is a targeting signal?
- could be at the N terminal and just linear (primary amino acid sequence)
- formed by 3D folding of the protein
- would be important for targeting specialised organelles
what could be indicative of targeting the nucles?
a lot of positive charge
what could be indicative of targeting the ER?
a lot of hydrophobic amino acids
what signal would proteins destined to be secreted or integrated in the ER?
have N terminus signal sequence 6-12 hydrophobic amino acids
what is the Signal Recognition Particle (SRP)?
recognises the signal sequence of Nascent polypeptide
- cystolic ribonucleoproteins of proteins bound to RNA scaffold
how does the signal allow entrance to the ER?
- SRP docks with SRP receptor on cytosolic face of ER
- the interaction triggers GTP hydrolysis and releases SRP
- GDP and Pi release
- on SRP release nascent polypeptide enters translocon membrane channel
- as it grows nascent polypeptide is fed through translocon in the ER (translocates through a hydrophillic pore)
- signal sequence is removed inside ER by signal peptidase enzyme
- protein is released into the ER lumen
what happens in co-translational import?
proteins to be tarrgeted to the ER initially have an N-terminal peptide, the ER signal sequence, translated by a cytosolic ribosome
what is the ER signal sequence bound by?
- an SRP
how does SRP cause translational arrest?
- polypeptide has an ER signal which binds SRP
- arrests translation until it starts entering the sec61
- translation arrest gives the ribosome enough time to bind to the ER membrane
- means the polypeptide isnt released into the cytoplasm
- polypeptide does not fold before reaching the translocon wouldnt be able to go through the channel
what is the Sec61 complex?
- the translocon
- four subunits that form a channel
how is the Sec61 complex involved in translocation?
- polypeptide chain enters the ER through the translocon
what is post translational insertion?
- requires the sec 62 and the larger Sec63(71 and 72) complex
- the SRP is not involved
- protein folds up in the cytoplasm
- after translation its inserted into the ER
- requires molecular chaperones
- deposits BIP (a chaperone) molecules onto the translocating chain as it emerges from the ER lumen
how is the ER signal peptide cleaved?
- cleaved by signal peptidase
- after protein translation, Sec61 opens and releases the signal peptide into the membrane where its rapidly degraded
- production of a luminal protein can be secreted or targeted to the lumen of an organelle
what is the signal pepride?
can be a stop/start transfer sequences
how do stop/start sequences work?
- high quantity of hydrophobic amino acids are recognised by the cell
how do the proteins respond to the stop/start sequences?
- protein gets moved laterally and is released from the Sec61 (proteins can have internal signal peptides that can cause the protein to stop transfer through Sec61)
- signal peptide cleavage
- this is then the transmembrane protein (crosses the membrane once)
what happens if the signal peptide not at the N terminus?
- transported to the ER lumen
- N terminus in the cytoplasm
- continue to make the C terminus
- type II membrane is created (SNARE proteins)
- multiple stop-transfer sequences results in polytopic membrane protein
what does a 19-21 hydrophobic region indicate?
- a transmembrane region
- hydrophobic side chains stick out of a alpha helical coil
what happens if it is a transmembrane protein?
- translocation halted
- signal peptidase cleaved
- carries on making the rest of the protein in the cytoplasm
what happens if you have internal start-transfer sequence?
- its hydrophobic
- binds but continues to make the protein
- get a SNARE protein
what might membrane proteins have instead of an N terminal transfer sequence?
the start transfer can be internal
what will the SRP scan for in a polypeptide?
for a region of hydrophobicity
what will sequences contain?
- a start sequence and then a stop sequence
- order of a start-stop sequence that determines membrane topology
which end of the transmembrane protein is the luminal end?
the cytosolic of TM domains is more positively charged than the luminal end (positive inside)
what do the majority of type II proteins have?
an internal sequence
what is a polytopic protein?
crosses the membrane multiple times
what is a type I protein?
NH3+ is on the lumenal side and COO- is on the cytosol side
what is a type II protein?
NH3+ is on the cytosol side and COO- is on the lumenal side
