Endoplasmic reticulum and secretory pathway
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Define a signal peptide
A short peptide (16-30 amino acids long) present at the N-terminus of the majority of newly synthesised proteins that are destined towards the secretory pathway
Describe the ER as a single-membrane compartment
Continuous network of tubular and flat vesicular structures in the cytoplasm (“little net”).
What is inside the compartment of the ER
A space which is connected with the space between the two membrane surfaces of the nuclear membrane (Continuous with the nuclear envelope)
Describe the two parts of the ER and briefly their functions
Granular/Rough ER (ribosomes attached (hence rough) for the translation and folding of new proteins) Agranular/Smooth ER - Synthesis of lipids and detoxification of certain drugs and toxins by cytochrome p450 enzymes
What do the RER and the SER look like on an electron micrograph
RER - Long tubular lines covered with black spots on them representing the ribosomes SER - Quite smooth and flat tubular structure, with no black spots (no ribosome)
State the functions of the RER
Protein synthesis, glycosylation, folding and assembly and multi-protein complexes
State the functions of the SER
Lipid synthesis (cholesterol, phospholipids) important for cell membrane for example. Doesn’t synthesise proteins Ca2+ sequestration - Important in contraction of muscle cells Detoxification by cytochrome P450 enzymes - Catalyse reactions that make drugs lipid soluble and waste water soluble to expel from the body
Name some differences between the RER and SER
Ribosomes v No ribosomes Found near the nucleus v Found closer to the cell membrane Originates from nuclear membranes v Originates from rough ER by giving off the ribosomes Mainly composed of cisternae vs Mainly composed of tubules Involved in synthesis, folding and transport of proteins v Synthesis and transport of lipids Well developed in protein forming and secretory cells v Mainly present in lipid forming cells
What happens in the first step of SRP
Recognition of the signal peptide - Ribosome
What happens in the second step of SRP
When the SRP particle binds the ribosome-nascent-chain complex, for this one GTP molecule is needed to provide energy for the process.
What happens in the third step of SRP
Binding of the complex composed of ribosome-nascent protein + the SRP particle + GTP to the SRP receptor. Two other GTP molecules are needed.
What is the initial synthesised form of insulin?
Preproinsulin
What happens when the insulin peptide is inside the RER
The first modification occurs, where the preproinsulin is brought to proinsulin and consists of a proteolysis, where the signal sequence is removed
What would the second modification be
Cleavage of the proinsulin into the three different peptides, the A, B and C chains
What happens to the A, B and C chains in the third modification
Disulfide bonds between A and B chains, C chain is removed to form a a biologically active insulin that comprises 51 amino acids.
What happens in the fourth step
The insulin goes through more modifications, the additions and removals of sugar/protein molecules, glycosylation, deglycosylation
Where does glycosylation/deglycosylation occur
Golgi
Why is glycolysation/deglycosylation important
Sorting secreted proteins (E.g. lysosomal proteins to lysosomes)
Where does protein folding and assembly occur?
Golgi
What happens to proteins that fail a quality check during protein synthesis?
They will not be exported from the ER. They will be degraded by ubiquitination and the proteasome. ERAD - Endoplasmic-reticulum-associated-protein degradation
Describe the golgi structure
Single-membrane compartment consisting of a 4 to 8 stacked layers of thin, flat, enclosed vesicles (cisternae) lying near one side of the nucleus
Name and describe the 3 networks that make up the golgi
• cis (First cisternae structure, closer to nucleus) • Medial and trans compartments (Final structure closer to cell membrane)
How does the golgi aid in protein modification
• Glycosidases, glycosyltransferases • O-linked glycosylation • Sulfatases • Proteases
How does golgi aid in lipid Synthesis
SPhingomyelin Glucosylceramide