The Endoplasmic Reticulum and Translocation Flashcards
Name the structure of membrane-enclosed organelle ?
- Mitochondrion
- Endosome
- Golgi Apparatus
- Lysosome
- ER reticulum with membrane-bound ribosomes
- Cytosol
- Peroxisome
- Nucleus
- Plasma Membrane
- Free ribosomes
Membrane-enclosed organelles import proteins by one of three mechanisms
- Transport through nuclear pores
- Transport across membranes
- Transport by vesicles
Briefly explain the Endoplasmic Reticulum ?
- Reticulum: a network of membrane tubes
- Plus outer nuclear envelope
- Closed membrane: contents separate from cytoplasm
Explain the ER structure ?
- The ER have ‘cisternae’ which are long, flattened, sac-like, unbranched tubules with a diameter of 40- 50 um.
- The fluid filled interior is generally referred to as the ER lumen but also as the cisternal space.
- The lumen is where the biochemical reactions occur and the modification processes required for proteins.
- On the rough ER (RER) ribosomes are dotted on the outside showing how closely linked this organelle is to protein synthesis
The RER is important for the ?
Secretory pathway and protein biosynthesis – folding and regulating misfolded proteins and responding to cellular stresses
Tissue types that require a lot of proteins to be manufactured and secreted contain cells that are ?
Necessarily high in RER
The main functions of SER are?
The biosynthesis of phospholipids and cholesterol and the synthesis and repair of membranes
In hepatocytes the SER contains large amounts of ?
Cytochrome P450 and participates in the detoxification of metabolic waste products, drugs, and alcohol
In myocytes the SER is known as ?
The sarcoplasmic reticulum and stores and releases calcium ions needed for muscle contraction
What is the Secretory Pathway also known as ?
The endomembrane system
Proteins destined for the Golgi apparatus, endosomes, lysosomes, as well as proteins destined for the cell surface all first enter the ?
Endoplasmic Reticulum (ER)
Explain the Rough Endoplasmic Reticulum ?
- Black dots are ribosomes
- They are making membrane and secreted proteins
- These proteins are translated and translocated at the same time
ribosomes targeted to ER membrane by ?
A signal sequence
N terminal signal sequences are required and sufficient for?
Cellular destination determination
What forms the translocon ?
The Sec61 complex (eukaryotes) Sec YEG (prokaryotes)
What forms the channel ?
The central Sec61α subunit (SecY); two smaller peripheral subunits are Sec61β and Sec61γ (SecE)
What does the translocon structure have ?
Transmembrane domains that span the ER membrane
Describe the inside of the channel ?
It is hourglass-shaped, with a ring at the center consisting of six bulky hydrophobic amino acid residues (the pore ring) which position their side chains to the center of the pore
What does the ring prevent ?
Leakage of ions through the inactive channel and during translocation of a protein substrate
What is the plug domain ?
The lumenal side of the closed channel is occupied by a short helix
ER signal sequence contains?
A short region of hydrophobic amino acids, usually at N-terminus
What does Signal Recognition Particle (SRP) do?
- Binds to the signal and the ribosome
- stops translation
- Then it binds the SRP Receptor in the ER membrane
SRP receptor recruits the ?
Translocation channel and translation continues
Soluble proteins are translocated across ?
The membrane and released into the ER lumen
Signal sequence is cleaved from ?
The protein by signal peptidase
Explain the start-transfer and stop-transfer sequences in single-pass transmembrane protein ?
- Single-pass transmembrane protein: an N-terminal signal sequence starts transfer into the ER
- A second, longer hydrophobic sequence (~20 a.a.s) stops the transfer
- The rest of the protein is synthesised in the cytoplasm
Explain the start-transfer and stop-transfer sequences in multi-pass transmembrane protein ?
- Multi-pass membrane protein: internal hydrophobic sequence bound by SRP: starts translocation into the ER
- ..but not a substrate for signal peptidase
- Next hydrophobic sequence stops translocation
- The rest of the protein is synthesised in the cytoplasm
N terminal signal sequences determine ?
Targeting of proteins to RER and subsequent translocation into RER
Co-translational translocation requires ?
SRP and Sec61 translocon
Post-translational translocation requires ?
Hsp70 (Ydj1) and additional proteins to Sec61 (Sec62, Sec63, Sec71 and Sec72) as well as luminal chaperones to ratchet in soluble proteins)
