Lecture 9: Intro to the ER Flashcards
Rough ER
- Ha ribosomes bound on the cytosolic membrane surface
- Composed of a network of cisternae
- continous with the outer membrane of the nuclear envelope
Endoplasmic Reticulum (ER) (2)
what it is+divided into
- A system of membranes and vesicles that encloses the ER lumen (space inside the ER, seperate from cytosol)
- Divided into smooth ER and rough ER
Smooth ER (3)
- Lacks ribosomes
- Composed of interconnected curved tubular membranes
- continous with the RER
Smooth ER is extensive in cell types such as (3)
- skeletal muscles, kidney tubules and steroid-producing endocrine glands
Functions of smooth ER (4)
- synthesis of steroid hormones (ex: estrogen)
- Synthesis of membrane lipids (ex: phospholipids)
- Detoxification of organic compounds in the liver
- Sequestering calcium ions (storage site for calcium within cells and released with signal) in skeletal and cardiac muscle- role in muscle contraction
Function of rough ER (2)
- Protein synthesis
- Modification of protein is started here (addition of sugars)
Rough ER is entensive in
- Extensive in cells with a role in protein secretion (ex: intestinal cells that secrete mucoproteins)
Sites of protein synthesis (2)
- 1/3 of proteins in Rough ER
- 2/3 of proteins in free ribosomes
Rough ER ribosomes protein synthesis (3)
- Co-translational translocation
- Secreted proteins (Once inside the lumen of the RER, these proteins undergo various modifications, such as folding, glycosylation, and disulfide bond formation, to attain their functional conformation. After processing in the ER, secreted proteins are transported to the Golgi apparatus for further modification, sorting, and packaging into vesicles for secretion via exocytosis.)
- Integral membrane proteins and soluble proteins that reside in compartments of the endomembrane system (The rough endoplasmic reticulum (RER) is also responsible for synthesizing integral membrane proteins and soluble proteins that reside within various compartments of the endomembrane system, including the ER itself, the Golgi apparatus, lysosomes, and vesicles.)
Co-translational translocation
synthesis of secreted proteins, soluble proteins and integral membrane proteins that reside in endomembrane
Free ribosones protein synthesis (3)
Made by+released+what kind?
- Synthesized by free ribosomes in the cytosol (not attached to the ER)
- Proteins are released into the cytosol
- Makes proteins that remain in the cytosol, peripheral proteins of the cytosolic surface of membranes (weakly attached to plasma) and proteins transported to the nucleus, mitochondria and chloroplast (peroxisms)
Co-translational translocation steps for secreted and soluble proteins:
- Begins on a free ribosome
- Signal sequence (directs ribosome to membrane) usually at N-terminal end is 6-15 hydrophobic amino acids
- Signal secognition particle (SRP) binds to the signal sequence and the ribosome
- Polypeptide synthesis is halted temporarily
- SRP directs this complex to the ER membrane by interaction with the SRP receptor (ribosome+peptide)
- Ribosome/polypeptide then are transfered from the SRP once it docs to the receptor to the translocon (a protein pore in the ER membrane with a plug to prevent ion diffusion)
- Contact with the signal sequence displaces the plug
- SRP released from the SRP receptor
- Translocation through the pre: Polypeptie enters the ER lumen
- Upon termination, ribosome is released
- Signal sequence is removed by an enzyme: signal peptidase
- Protein chaperones (Eg: BiP) aid in protein folding
Co-translational translocation steps for integral membrane proteins:
- Synthesized by co-translational translocation using the same machinery as secreted proteins (SRP, receptor etc)
- SRP recognizes the hydrophobic transmembrane domain as the signal sequence
- Transmembrane domains do not pass through the pore-instead, they directly enter the lipid bilayer
- As polypeptide pass through the translocon, a gate in the pore opens and allows the proteins to partition themselves according to their solubility properties (Either in the aqueous pore or in the hydrophobic lipid bilayer
Co-translational translocation for integral membrane proteins the directionality of the insertion
Direction of insertion into the bilayer is dependent on:
1. Positively charged amino acids flank the cytosolic end of the transmembrane domain.
2. Cytoplasmic leaflet is more abundant with PS and PI phopholipids which are negatively charged
If the positive charges are on the N-terminal side of the transmembrane domain….
The translocon will reorient the transmembrane domain
