Endomembrane I Flashcards
Describe a fundamental requirement of all cells is
- ability to selectively exchange materials across their plasmamembrane for hospitality
- bidirectional absorption and secretion
- so that cells can communicate with their environments
Describe absorption
exchanging into the cell from the environment
Describe secretion
releasing into the environment
How is bidirectional exchange across plasmamembranes facilitated?
specialised transporter proteins that are inserted into the membrane
Describe prokaryotic protein secretion
- involves a signal peptide
- preceded by positively charged residues
- removed by a specific protease post-secretion.
Describe the signal peptide involved in prokaryotic protein secretion
- very hydrophobic
- approximately 20 amino acids
- approximately a bilayer length
- coded within the protein sequence
- usually localised at the N-terminus
Describe the basics of the interaction of the signal peptide and the SRP
- facilitates nascent protein co-translational translocation across the membrane barrier
SRP
Signal Recognition Particle
Describe the SRP
ribonucleoprotein masks the hydrophobicity of the signal peptide
Describe the specifics of the interaction of the signal peptide and the SRP
- when the SRP recognises the signal peptide, it arrests translation by capping the exit, and recruits the peptide to dock onto the protein translocation channel in the plasmamembrane
- signal peptide becomes stuck in the channel due to its hydrophobicity
- laterally diffuses from the membrane
- hydrophilic regions are pushed out through the membrane, into the extracellular space
- once ribosome becomes attached to the channel, SRP dissociates
- ribosome is released
- translation resumed
Describe membrane channel specificity
- different proteins will have different specific channels
- e.g. all proteins with TND will have to use the YMD channel
What happens after translation is resumed?
- signal peptide is cleaved by proteases
- remaining stretches of mRNA are translated and localised in the extracellular space
Describe ‘stop-transfer’ and ‘restart-transfer’ signals during synthesis.
- some transmembrane proteins can span the membrane several times because of their hydrophobic TMDs
- they become caught in the lipid bilayer, and the proceeding and preceding sections form convolutions on the outer and inner sides of the membranes
TMD
transmembrane domain
Describe the relationship between membranes and proteins in prokaryotes
Since there is only one membrane, this is where all of the membrane proteins reside, and all of the proteins that reside in the membrane carry out its membrane functions.
Describe tradeoffs in prokaryotic membrane systems
- SA:Vol of prokaryotes imposes important geometrical constraints on their size and shape
- diffusion-limited processes become limited towards the centre of the cell
- demand of the cytoplasm for ATP and nutrients outweighs the ability of the membrane to create this ATP and transport this nutrients
Give some examples of diffusion-limited processes in prokaryotes
- nutrient uptake
- gaseous exchange
Describe the results of the tradeoffs in prokaryotic membrane systems
maintenance of a small, rod-like shape, where the centre of the cell can never starve.
Describe the eukaryotic innovation to overcome tradeoffs in membrane systems
- adapt their prokaryotic ancestral membranes into homologous intracellular membranes
- highly specialised internal membrane-bound organelles, and an internal membrane system, that allow for the evolution of larger cells, greater complexity, and subfunctionalisation
Describe the endomembrane system
- creation of disparate but specialised and optimised conditions for enzymes. - approximately 10-20% (1500-6000 genes) of eukaryotic genes are directly involved in management of the endomembrane
Describe the single endomembraned organelles
- involved in import and export of macromolecules
- of endogenous origin
List some of the single endomembraned organelles
- rough ER
- Golgi body
- endosome
- secretory vesicles
- lysosome
Describe the origins of the endomembrane
- ancient anaerobic archaeon cell that would ultimately derive the eukaryotic lineage contained genomic DNA, a plasma membrane and a rigid cell wall
- loss of this cell wall would facilitate HGT, which would occur via the phagocytosis and digestion of other archaeal and bacterial prokaryotes
- acquisition of new genes rapidly would speed up the evolutionary rate of the archaeon
- selective pressure for membranous enclosure of the chromosome for protection
- endosymbiogenetic uptake of the facultative anaerobe alpha-proteobacteria to form a protomitochondrion would lead to nuclear envelope development
- as multiple mitochondria are acquisitioned, more energy is available for evolution of the full membrane system and cell growth, birthing the first aerobic eukaryotic cells
Describe LECA
contained a basic set of endomembrane compartments, comprising the ER, endosomes, lysosome, vesicle transport and phagocytosis, but not the Golgi or vacuole.
Describe the advantages of the multi-membraned eukaryotic cell
- compartmentalisation of its cellular internal membrane-bound organelles towards specialised cellular functions
- addition to the membrane surface area for reaction time
Describe the basics of mRNA export
- to be exported from the nucleus to the cytoplasm, the mRNA must cross the nuclear envelope using active transport across selective nuclear pore complexes
- involves a signal peptide and SRP.
What is the nuclear envelope?
the nuclear double membrane
Describe the specifics of mRNA export
- SRP receptor in the membrane of the rough ER
- receptor docks the ribosome (which is bound to the SRP) onto the membrane, allowing interaction with the protein translocation channel
- allows translation to continue and translocation to begin
- SRP and its receptor are displaced and recycled
Describe the rough ER
- controlled environment
- studded with ribosomes
- involved in protein and lipid synthesis, protein folding and modification (including glycosylation), as well as active protein translation and translocation and storage during quality control
- continuous with the nuclear envelope’s outer membrane
- site to which secrete and plasmamembrane proteins are delivered
Describe the relationship between eukaryotic and prokaryotic translocation channels
Homologous
What else happens at the SRP of the rough ER in eukaryotic cells?
glycosylation of secreted and plasmamembrane proteins
Describe glycosylation of secreted and plasmamembrane proteins
- N-linked oligosaccharide glycans are added to Asparagine side chains (coupled with a nitrogen atom in certain Asparagine residues)
- if the protein is properly folded, the glycan be processed
- soluble and transmembrane proteins can then be exported from the ER via packing into transport vesicles, budding off of the lumen
- trafficked to the plasmamembarne or extracellular space in search of the target compartment for fusion
What are the implications of glycosylation?
signalling and protective functions, as well as quality control
What happens to incorrectly folded proteins during glycosylation?
- bound to chaperone proteins.
Describe the vesicles used by the ER post-glycosylation
- range in diameter from 30-100nm
- wrapped in subunits of COPII coating
- use the ER as a donor compartment
Describe the smooth ER
an interconnected compartment containing diverse proteins that is the site of cellular lipid and sterol synthesis.
Which cells have more rough ER
protein secreting cells such as the digestive-enzyme secreting pancreatic acinar cells
Which cells have more smooth ER
steroid synthesising cells
Describe the trafficking of vesicles containing glycolsylated products
first destination is the Golgi apparatus,
Describe the basics of the Golgi apparatus
- a functionally compartmentalised stack of flattened membrane sacs known as cis-, medial- and trans-cisternae cisternae containing disparate enzymes
- named after its observer Camillo Golgi (1898).
Describe mammalian Golgi stacks
found in a large, continuously cluster titled the Golgi ribbon, near the nucleus
Describe plant, fungal or invertebrate Golgi stacks
generally more than 200 small, mobile stacks will be distributed throughout the cell
The Golgi apparatus has its principal functions in
- carbohydrate, glyco- and sphingolipid synthesis
- protein and lipid processing and modification
- cargo sorting: trafficking of lysosomal and vacuolar enzymes to the late endosome, and thence to their final organelle destination.
Why is the Golgi termed the ‘polysaccharide factory’,
processing the N-linked glycans on glycoproteins through sequential cisternae in the lumen, and synthesising polsyachharide cell components.
Describe the cis-cisternae
- the first cisternae
- site of protein and lipid arrival
- necessary for the phosphorylation of oligosaccharides on lysosomal proteins, and the removal of Man
Describe the medial-cisternae
- phosphorylation of oligosaccharides on lysosomal proteins, and the removal of Man
- addition of GlcNAc
Describe the trans-cisternae
- add Gal and Nana
- complete sorting into the secretory vesicles for further trafficking towards the plasmamembrane
- trimming the glycoprotein to a simple common core
- assembly of a diversity of complex branches onto it, so that the diversity of possible reactions in the extracellular space is increased
Describe the regulation of protein secretion
directing the proteins into specialised secretory vesicles (lipids and polysaccharides are often able to share the same vesicle)
Describe protein secretory vesicles
- approximately 1 micrometer
- require signal reception before plasmemembrane fusion
Give examples of protein secretion
- insulin in the pancreatic beta-cells
- hydrolase in pancreatic acinar cells
Describe the endocytic pathway
- selective internalisation of macromolecules via specific plasmamembrane receptor proteins from the external environment across the plasmamebrane and into the cytoplasm
Secretion is usually
immediate
Internalisation can be achieved via multiple mechanisms depending on
the size of the entity being taken up
Describe phagocytosis
- engulfment of bacteria of 1micrometer or more
- bacterium is phagocytosed into the phagosome, which transports its cargo to the lysosome and fuses there for digestion
Describe the endocytosis of small endocytic vesicles of 0.5micrometers or less
- specific receptor initiates vesicle formation
- receptor-cargo complex is internalised into a vesicle
- trafficked to the early endosome, where the two subunits are separated: receptor is repackaged and recycled via the recycling endosome where it is returned to the membrane to avoid degradation, and the cargo is sorted and transported further down the endocytic pathway
- endosome matures into the late endosome, which fuses to activate enzymes
- continues trafficking towards digestion and the lysosome or vacuole
Describe the late endosome
where all vesicles packaged from the secretory diffusion pathway of the Golgi containing non-discriminative lysosomal cargo degradation enzymes, and the endosomes, converge and fuse.
What are endocytic pathways used for?
- macromolecules such as nutrients, proteins and signalling molecules
- cell and pathogen endocytosis
Describe the lysosomes
- site of cellular digestion
- acidic and hydrolytic, containing digestive enzymes
Describe the digestive enzymes present in the lysosomes
- proteases
- lipases
- glycosidases
- only active at a low pH
As the endocytic pathway progresses, it becomes increasingly …
acidified.
Describe prokaryote digestion of macromolecules
- external
- secrete enzymes and transport useful digestion products into the cytoplasm
- relies upon diffusion, and therefore lacks efficiency
Describe eukaryote digestion of macromolecules
- digest their internalised cargo in hydrolytic compartments
– a more efficient method - toxic products can be packaged and removed
Describe plant and fungal hydrolytic vacuoles
- used for digestion, because the presence of their cell walls preclude phagocytosis, but not endocytosis
How much of a plant or fungal cell is taken up by a vacuole?
90%, enlarged via tubes and sheets as well as water accumulation.
