Membrane Proteins Flashcards
(34 cards)
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
