The endomembrane system I Flashcards
(35 cards)
Summary
Advantages of cellular compartmentalisation
ER, Golgi, EE, LE, RE, lysosome/vacuole
Specialisations for lipid, protein, and polysaccaride synthesis and degradation
Transport on secretory and endocytic pathways
Why are cells described as self organising molecular assemblages
No blueprint, no template, no central organiser
Order and organised dynamics spontaneously arise from individual interactions between hundreds of molecules
Secretion of proteins in prokaryotes
All proteins made on cytosolic ribosomes and signal peptide within protein sequence - very hydrophobic and preceded by positively charged residues and removed by specific protease after secretion
Please describe the role of the signal recognition particle
SRP recognises the signal peptide and translation arrests
SRP promotes association with a protein translocation channel and SRP dissociates
Ribosome is attached to channel and translation resumes
Nascent protein is translocated co-translationally
Signal peptide is cleaved by proteases
How does SA:V impose geometrical constraints on size and shape
Diffusion-limited processes become inefficient such as nutrient uptake and gas exchange
Demands of cytoplasm for ATP and nutrients exceed capacity of PM to service demand
Therefore, must stay small and rod shaped
How are transmembrane proteins that span the membrane several times made?
Hydrophobic stretches (transmembrane domains) are interpreted by the translocation channel as ‘stop transfer’ and ‘restart transfer’ signals during synthesis
Briefly outline the origins of internal membrane-bound compartments
Mitochondria (α proteobacteria) chloroplasts (cyanobacteria) – endosymbiotic origin due to double membranes and energy transduction
Endomembrane organelles have an endogenous origin - single membranes and import and export of macromolecules
Mechanism of SRP and secretion
Almost identical to prokaryotes
Signal peptide and SRP. SRP receptor in the ER membrane helps the ribosome to dock onto the membrane and interact with a protein translocation channel
Translocation channel is homologous to the channel at the eukaryotic PM
What kind of proteins are delivered to the ER
Secretory and plasma membrane proteins
Describe how many proteins are modified by glycosylation
Starts in the ER
Addition of N-linked glycans to proteins in the ER
Coupled by a N atom in certain Asp residues
Glycan processing is completed only if protein is properly folded (signalling functions, quality control, protective functions)
Protein export from the ER
- Only correctly folded proteins are permitted to leave
- They are packaged into transport vesicles that bud off from the ER
- Transmembrane proteins and soluble proteins travel together
Proteins leave the ER in COPII-coated transport vesicles
Those destined for the Golgi and beyond are packaged into COPII transport vesicles and they bud from ER exit sites (dispersed throughout ER network).
Cargo membrane proteins display exit signals on their cytosolic surface that adaptor proteins of the inner COPII coat recognise. The exit step from the ER is a major quality control place.
Only correctly folded proteins can leave the ER
What happens to those that are not properly folded?
To exit they must to be properly folded and if they are sub united, must be fully assembled. Those who don’t fit the criteria remain in the ER bound to chaperone proteins such as BiP or calnexin. Chaperones may cover the exit signals, and then proteins are taken to cytosol to be degraded by proteasomes. This is a quality control step that prevents malformed proteins from interfering with the function of normal proteins.
Describe the structure of the Golgi apparatus
Collection of cisternae - flattened membrane enclosed compartments. Usually 4-6 cisternae and can be connected by tubular connections (microtubule)
Difference of Golgi apparatus in plants and mammals
Plants have many small (over 200) Golgi stacks while mammals have a single large clustered Golgi stack near the nucleus.
The Golgi stack is functionally compartmentalised
Cis, medial, and trans cisternae all harbour different sets of enzymes
Polysaccharide synthesis occurs progressively in sequential cisternae
What is the CGN and TGN
Cis Golgi network is a collection of fused vesicular tubular clusters from the ER. Proteins entering the CGN can proceed in Golgi or go back to the ER. Proteins exiting from the TGN are sorted to they next destination - endosomes, secretory vesicles, or the cell surface (or retuned to an earlier compartment). Proteins are modified in successive stages as they move through the cisternae.
What are the two main classes of N-linked oligosaccarides
Complex oligosaccardies and high mannose oligosaccharides.
Complex ones are generated when original N-linked oligosaccharide is trimmed and further sugars are added
high mannose ones are trimmed and have no new sugars added
What are three classes of proteins before they leave the TGN?
Those going to
Lysosomes
Secretory vesicles
Cell surface (called the default pathway because no particular signal required)
Secretory vesicles bud from the trans Golgi network
Secretory vesicles form from the TGN and carry out exocytosis (in response to specific signals)
Signals that direct secretory proteins to secretory vesicles are not well defined - unclear how agreggates of secretory proteins are segregated into secretory vesicles
Secretion of some proteins can be regulated
- Secretory proteins are directed into specialised secretory vesicles
- Vesicles fuse with PM only after a signal is received
How is the secretion of some proteins regulated?
Once loaded, a secretory vesicle has to reach the site of secretion. Secretory vesicles in the regulated pathway must wait at the membrane until the cell receives a signal to secrete
Signal can be an action potential or a hormone (extracellular signal) in both cases it leads to a transient increase in the concentration of free Ca2+ in the cytosol.
What about transport vesicles containing materials for constitutive release
These transport vesicles simply fuse with the PM once they arrive there
Transport into the cell from the plasma membrane - endocytosis : what is it for?
What are the three types of macromolecules transported into the cell
Transports macromolecules from the external environment to lysosomes/vacuole for digestion
- Nutrients (proteins, cells)
- Signalling molecules
- Pathogens
