The Endomembrane System Flashcards
How do prokaryotes secrete proteins
Through signal peptides (usually at the N-terminus) that are ~20 hydrophobic amino acids long and often preceded by positively charged residues. These peptides guide secretion and are usually cleaved post-translocation
What is the role of the SRP in prokaryotic secretion
- SRP (Signal Recognition Particle) recognizes the signal peptide, arresting translation.
- SRP directs the ribosome to a translocation channel.
- SRP dissociates, translation resumes, and the protein is co-translationally translocated.
- The signal peptide is cleaved by a protease after translocation
How are hydrophobic stretches of proteins interpreted during synthesis
They are interpreted by the translocation channel as start-transfer or stop-transfer signals, guiding insertion into membranes
Why are prokaryotic cells often small and rod shaped
Their single membrane must fulfill all functions, and SA:V constraints limit their size. Small, rod-shaped cells maintain efficient diffusion and energy/nutrient exchange
What is the endomembrane system and its function
A collection of membranous organelles (ER, Golgi, vesicles, lysosomes) that handle synthesis, transport, and degradation of proteins/lipids. It increases membrane surface area and creates compartmentalised microenvironments
What is the origin of endomembrane organelles
They are derived endogenously from the plasma membrane
What are key roles of the plasma membrane
Interacts with the external environment, handles material exchange, and serves as the cell boundary
How are macromolecules imported/exported across the nuclear envelope
Via nuclear pore complexes, which are selective, ATP-dependent channels
What happens to secreted and membrane proteins in the ER
They enter the rough ER, where translation and translocation occur. Proteins are folded and glycosylated (N-linked glycans on Asn residues)
What is the difference between rough and smooth ER
Rough ER: ribosomes attached; site of protein synthesis/folding.
Smooth ER: lacks ribosomes; synthesises lipids and sterols
How are proteins transported from the ER to the Golgi
In COP-II vesicles, which bud from the ER and carry only correctly folded proteins
What are the key roles of the Golgi apparatus
- Trimming and processing N-linked glycans.
- Synthesising glycolipids and sphingolipids.
- Functionally compartmentalised into cis, medial, and trans cisternae for sequential processing.
Is a signal required for ER export
No specific signal is required - bulk flow can export proteins by default. However, signals are required to retain proteins in the ER
What are ER retention/retrieval signals
Short amino acid sequences (e.g. KDEL for soluble proteins, KKXX for membrane proteins) that are both necessary and sufficient to retain proteins in the ER
How are ER resident proteins retrieved
Via COP-I vesicles, which recognize retrieval signals and return proteins from the Golgi to the ER
How are Golgi-resident proteins localised
Through their transmembrane domain (TMD) length and cytosolic tail, which match the Golgi membrane thickness and signal localisation
How do membrane thicknesses differ across organelles
Plasma Membrane > Golgi > ER
Proteins match their TMD length to the membrane thickness for proper localisation
What are the four fates of proteins in the Golgi
- PM/secretion (default)
- Lysosome via CCV (signalled)
- Return to ER via COP-I (signalled)
- Retention in Golgi (signalled)
What is the function of the endocytic pathway
Imports macromolecules from the environment to lysosomes or vacuoles for digestion
What are the main internalisation mechanisms
- Phagocytosis (engulfment of large particles)
- Endocytosis (small vesicle internalisation)
How are receptors and cargo handled in endocytosis
Receptor and cargo bind at the PM and are internalised.
Travel to early endosome (EE).2
Receptors are recycled; cargo progresses to late endosome (LE) and lysosome.
What are CCVs and how are they formed
Vesicles with two layers:
1. Clathrin (outer): bends membrane.
2. Adaptin (inner): selects cargo via receptor tails
How do LDL particles enter cells
Via LDL receptors into clathrin-coated pits, forming CCVs that go to the early endosome where LDL is released
How are lysosomal hydrolases made and targeted
Made in the ER as inactive pro-enzymes.
Modified in the cis-Golgi (mannose-6-P tag via signal patch).
Sorted by M6P receptor into clathrin-coated vesicles with AP1 adaptin to go to the late endosome.
What happens to lysosomal hydrolases in the late endosome
M6P receptor releases cargo due to low pH.
Hydrolases become active via proteolytic cleavage in the lysosome
How does digestion differ between prokaryotes and eukaryotes
Prokaryotes: Secrete enzymes for external digestion, then import products.
Eukaryotes: Internalise macromolecules via endocytosis and digest internally in lysosomes
What determines whether a protein remains in the ER or is secreted
ER-resident proteins have specific retrieval signals (e.g. KDEL or KKXX) that are necessary and sufficient to retain them; without the signal, they are secreted by default
What is the role of COP-II vesicles
COP-II vesicles mediate export from the ER to the Golgi, carrying properly folded proteins and lipids
How does membrane thickness relate to protein localisation
The increasing thickness from ER → Golgi → PM correlates with increasing TMD length of membrane proteins, determining their localisation
What is the functions of adaptins in the CCVs
Adaptins form the inner layer of CCVs and recognise cytoplasmic tails of cargo receptors, ensuring selective cargo loading
What triggers cargo release from receptors in the endosome
A drop in pH in the early endosome causes conformational changes in receptors, leading to cargo release
What is a ‘signal patch’ and what does it do
A signal patch is a 3D structural feature on lysosomal hydrolases that is recognised by cis-Golgi enzymes, which then phosphorylate a mannose residue, targeting the protein for lysosomal delivery
What is the role of the NSF complex in vesicle fusion
NSF disassembles SNARE complexes after vesicle fusion, allowing SNAREs to be reused in future trafficking steps
What is the function of the Golgi’s trans face
The trans face is the exit side of the Golgi, where proteins are sorted and packaged for delivery to the PM, lysosomes, or back to the ER
How are Golgi-resident enzymes retained in the Golgi
The trans face is the exit side of the Golgi, where proteins are sorted and packaged for delivery to the PM, lysosomes, or back to the ER
Why is the lumen of the ER considered topologically equivalent to the extracellular space
Because proteins in the ER lumen do not need to cross another membrane to be secreted - they are already in the same orientation as the extracellular environment
What is the ‘bulk flow’ hypothesis for ER export
Most proteins don’t require a specific export signal to leave the ER - they exit by default unless retained by a signal
What experimental evidence supports the bulk flow model of ER export
Bacterial proteins lacking export signals are secreted when tagged with a eukaryotic signal peptide and expressed in eukaryotic cells, showing that export doesn’t require a specific signal
How does membrane lipid composition affect organelle identity
Differences in lipid composition affect membrane thickness, which in turn influences where membrane proteins with different TMD lengths localise (e.g. PM > Golgi > ER)
How is TMD length used to localise proteins to specific organelles
TMD length is matched to membrane thickness - shorter TMDs stay in thinner membranes like the ER, while longer ones are sorted to thicker membranes like the PM
What is the structure and function of the clathrin lattice
Made of clathrin triskelions, it forms a stable, curved cage-like lattice that drives vesicle formation and curvature at the PM and Golgi
How does AP1 and AP2 adaptin complexes differ
AP1 operates at the trans-Golgi for lysosomal targeting, recognising Man-6P receptors; AP2 functions at the PM, helping form endocytic vesicles (e.g. for LDL uptake)
How are Mannose-6-Phopahte
They release cargo in the late endosome and are recycled back to the trans-Golgi via transport vesicles
How does receptor-mediated endocytosis ensure specificity
Receptors bind specific ligands at the PM, concentrate into coated pits, and are internalised via CCVs, ensuring only bound molecules enter the cell
