Protein and Membrane Trafficking Flashcards
List the major components of the endo-membrane system of eukaryotic cells.
Nuclear envelope
Endoplasmic reticulum
Golgi apparatus
Lysosomes - Contain digestive enzymes
Plasma membrane
Vesicles - Small membrane-bound sacs that transport materials
Peroxisomes - lipid metabolism and the detoxification of reactive oxygen species
Endosomes - sorting and transporting materials internalized by endocytosis
Define the major topological relationships between membrane compartments in eukaryotes:
Continuous compartments - nuclear envelope continuous with R.E.R
ER, Golgi apparatus, lysosomes, endosomes, and the plasma membrane are topologically distinct but are connected via vesicular transport
Cytoplasmic separation - cytosol is topologically distinct from the interior of membrane-bound organelles
Equivalence of lumen compartments to extracellular space
Mitochondria is topologically different containing their own genome, proteins must be imported via translocators
Describe Protein trafficking between cellular compartments:
Definition - directed movement of proteins between distinct membrane-bound compartments within the cell, often using vesicular transport.
Proteins are synthesised on ribosomes in the cytosol
Protein transport depends on amino acid sequences that contain sorting signals that directly deliver to locations outside the cytosol.
Most proteins do not have a sorting signal and consequently remain in the cytosol.
Other proteins have specific sorting signals that direct transport from the cytosol into the nucleus and other organelles.
It is functionally important that endosomal proteins are trafficked to the correct intracellular compartment.
Describe the process of gated transport:
Involves selective protein passage between cytoplasm and nucleus via nuclear pore complexes (NPCs) .
Bidirectional transport
Proteins synthesised in cytoplasm contain a nuclear localization signal (NLS), recognized by transport receptors (importins).
Larger molecules like proteins and RNA require active transport mechanisms.
Maintains nuclear-cytoplasmic compartmentalization critical for gene expression and cellular regulation
Define intracellular protein transport:
General movement of proteins within the cell from where they are synthesised (in the cytosol or the rough endoplasmic reticulum) to their functional destinations
encompasses all modes of protein movement
Describe the structure of a nuclear pore complex:
NPCs are composed of >30 different proteins, called nucleoporins.
Each pore complex contains one or more open aqueous channels through which small water-soluble molecules can passively diffuse
Ring structures provide stability
Central channel to allow RNA + proteins to pass through
NPC acts as a selective gatekeeper, allowing small molecules to diffuse freely while large proteins, RNA, and complexes require active transport mechanisms for gene expression and RNA processing.
Describe the process of transmembrane transport:
Proteins synthesised in the cytosol by free ribosomes contain specific signal sequences directed to their destination, recognised by receptors on the target organelle’s membrane called Signal Recognition Particles
Protein binds to a translocator which directly transport specific proteins across a membrane
The transported protein molecule must unfold to snake through the translocator.
Once inside the target organelle, the protein folds into its functional three-dimensional structure
Define the term exocytosis and roles within the cell
Exocytosis is the fusion of vesicles, derived from the secretory pathway, with the plasma membrane to expel the contents within.
Roles;
Secretion of molecules - hormones, enzymes, neurotransmitters
Membrane expansion and repair -
Plasma Membrane Protein Insertion
Removal of Waste and Cellular Debris:
Immune cells, like macrophages and neutrophils, use exocytosis to release antimicrobial substances or cytokines
What are the stages of exocytosis
1: Vesicle Tethering - involves connections made over distances more than roughly half a vesicle’s diameter from a specific membrane surface + uses SNARE proteins
2: Vesicle Docking - Holding of 2 membranes within a bilayer’s distance of one another, SNARE proteins use zipping technique
3: Vesicle Priming - molecular rearrangements and ATP-dependent protein and lipid modifications
4: Vesicle Fusion -
Merging of vesicle membrane with target
Driven by SNARE protein assembly and twisting
Results in release of large biomolecules into extracellular space
Describe calcium as a trigger for exocytosis:
Common trigger for exocytosis is a local rise in calcium ions
Binding of a neurotransmitter at one end of a huge neuron generates an action potential that causes remote triggering of exocytosis in a distant synapse of the same cell
the trigger is calcium influx via voltage-gated Ca2+ channels
Describe some mechanisms of inhibition by neurotoxins during exocytosis:
Botulinum toxin and tetanus toxin cleave or break apart the SNARE proteins
prevents neurotransmitter release causing muscle spasm
Describe vesicular transport:
Involves membrane-enclosed vesicles loaded with cargo molecules derived from the lumen of one compartment.
Vesicles are deposited in a second compartment by fusing with the membrane enclosing that compartment.
Transported proteins do not cross a membrane and so vesicular transport can only move proteins between compartments that are topologically equivalent.
The transfer of soluble proteins from the ER to the Golgi.
Describe the membrane dynamics of vesicular transport:
Budding - invagination of the membrane to create a pocket
Movement - cytoskeletal pathways, actin filaments
Fission - once the vesicle has formed, it must detach from the donor membrane, facilitated by dynamin
Describe membrane recognition in vesicular transport:
Membrane recognition by transport vesicles is mediated by a variety of phosphoinositide lipids (PIPs).
PIPs create a signature in the membranes and act as recognition sites for proteins involved in membrane remodelling
PIPs can be modified after fusion by enzymes located in the target membrane with the addition of phosphate groups by PI-kinases that can later be reversed by PI-phosphatases
