Protein Sorting II Flashcards
Common pathway for production and secretion of proteins.
1) Protein synthesis and translocation across ER membrane
2) Protein folding and modifications in the lumen of the ER
3) Protein transport to Golgi, lysosomes, or cell surface in vesicles
Organelles a protein might pass through on its way to it’s final destination
Starts with synthesis of polypeptides in the cytoplasm.
Can then go to the ER.
Then it can go through the golgi to Lysosomes or the plasma membrane.
Or it can be a cytosolic protein. It can have a targeting sequence that sends it to mitochondrion (or chloroplasts) or peroxisomes or the nucleus.
Pathway from ER to plasma membrane
Rough ER, Cis-golgi network, Cis-golgi, Medial-golgi, trans-golgi, trans-golgi network, secretory vesicle, exocytosed
ER signal peptides
typically at the N-terminus, (16-30 aa long,) Contain 6-12 hydrophobic residues (at their core) and are typically flanked by positive amino acids on one side or the other; otherwise their amino acid sequence is not conserved.
Most signal peptides removed in ER.
Translation is ____ in proteins targeting the secretory system.
Cotranslational
SRP
Signal recognition particle; a ribonucleoprotein complex, comprising 6 proteins and a single 300-nt RNA molecule
SRP binds to the SP on nascent polypeptide in the cytoplasm and stops translation until docked on the ER.
SRP also binds the large ribosomal subunit and the SRP-receptor on the ER
What does P-54 do?
Binds ER signal sequence
What does P68/P72 do?
It is required for protein translocation
What does P9/P14 do?
It interacts with ribosomes
SRP receptor
integral membrane protein, heterodimeric complex with alpha and beta subunits. SRP-receptor docks the SRP, ribosome, and nascent polypeptide onto the ER. GTP hydrolysis releases the SRP from the ribosome and allows continued translation and translocation into the ER
Both SRP-p54 subunit and the SRP receptor-alpha-subunit are _____.
GTP-binding proteins (but are not GTPases on their own)
ER translocon
Sec61alpha (makes a protein-lined pore in the ER membrane)
Sec61alpha
makes a protein-lined pore in the ER membrane, comprises 10 membrane spanning alpha-helices that form a pore. At the middle is a constriction or gasket lined with hydrophobic isoleucine residues.
The channel is gated by a peptide plug, so that it is only open during translocation.
The ribosome associates directly with the translocon and the polypeptide is translocated across the ER membrane in _____ form.
unfolded
Where does the energy for translocation across the ER membrane comes from ____.
polypeptide synthesis / translation
A ____ removes the signal peptide soon after translocation into the lumen of the ER.
signal peptidase
BiP
Binding protein;
molecular chaperone that facilitates protein import;
Hsc70 chaperone that uses ATP hydrolysis to draw in protein
Examples of Type I Integral membrane proteins
Glycophorin, LDL receptor, Influenza HA protein, Insulin receptor, Growth hormone receptor
Type I integral membrane protein
Contains a cleavable, N-terminal SP;
Contains 20-25 hydrophobic, transmembrane spanning alpha-helix;
Has second, STA (stop-transfer anchor) sequence;
So two total signals: Signal peptide and STA. Direct orientation
Examples of Type II integral membrane proteins
Asiaglycoprotein receptor, transferrin receptor, Sucrase-isomaltase precursor, golgi galactosyltransferase, Golgi sialyltransferase, Influenza HN protein
Type II integral membrane protein
Lacks N-terminal signaling peptide, but has an internal signal-anchor sequence.
The signal anchor functions as a signal peptide and a membrane anchor.
Positively charged amino acids adjacent to the signal peptide keep the N-terminus opposite to Type I proteins.
Type III integral membrane protein.
Lacks N-terminal signaling peptide, but has an internal signal-anchor sequence.
The signal anchor functions as a signal peptide and a membrane anchor.
Type III integral membrane proteins have positive amino acids on opposite side of transmembrane sequence than type II (adjacent to signal peptide).
Example of a Type III integral membrane protein
cytochrome p450
Example of a Type IV integral membrane protein
Sec61, Connexin, G protein-coupled receptors, Glucose transporters, Voltage-gated Ca2+ channels, ABC small molecule pumps, CFTR channel
