6: Protein Processing & Trafficking Flashcards
How does transport of proteins occur?
- Soluble proteins = cargo as they are carried inside a double membrane bound vesicle (in the lumen) and protected from the contents in cytoplasm
- Proteins with a signal (destined to be in the membrane of an organelle or on the plasma membrane) get embedded in the vesicle membrane = transmembrane protein, to be delivered to target membrane
Outline how proteins are transported from the donor membrane to the target membrane
Vesicles bud from specific regions of the donor compartment, identified by a specific coat protein. Coat protein is later discarded, allowing membranes of vesicle & recipient to interact and fuse
Give the names and functions of coat proteins
COP II = coats vesicles which transport molecules from ER to Golgi
COP I = coats vesicles which transport molecules from Golgi back to ER
Clathrin = coats vesicles which transport molecules between Golgi, lysosomes, and plasma membrane
Describe the structure of Clathrin
Contains subunits which each contain 3 large and 3 small polypeptide chains which together form a 3 legged triskelion.
Triskelions form a basket like convex framework of pentagon and hexagons
Explain the role of clathrin in transport
- Requires a second protein adaptor complex to attach the Clathrin coat to the membrane
1. Cargo receptor binds to cargo, causing a conformational shape change (both inside & outside membrane)
2. This allows adaptor protein to bind , and so clathrin coat can also be attached.
3. This then bends the membrane (when multiple clathrin units work together) = budding
4. Another protein, dynamin, pinches off the membrane by binding in a ring of the neck of the budding vesicle
5. The coat is rapidly released lost & recycled and naked vesicle is ready for transport
Describe the control of where vesicles dock
Vesicles have surface markers (=snare proteins) to identify them according to original and cargo
- V-snares when on vesicle
- T-snares when on target membrane
^ these are complementary to each other. When they meet they wrap around one another locking the 2 membranes together (=docking) and cargo is released into target membrane
Define exocytosis
The transport & fusion of secretory vesicles with the plasma membrane and the extracellular space. Requires energy in the form of ATP
Define endocytosis
Process by which cells absorb external material by engulfing it with the cell membrane
Why is exocytosis regulated?
Allows concentration of proteins to be released
Allows further processing of proteins
Allows release in response to a trigger
Describe the different types of endocytosis
Pinocytosis = ‘cell drinking’ - way of sampling things outside the cell
Receptor-mediated endocytosis = a way of taking in something from outside the cell e.g. a protein, through its binding to a protein ‘receptor’ in the plasma membrane
Phagocytosis = ‘cell eating’ - removing microbes from the body
Describe the process of pinocytosis
Clathrin coated pits on the cell surface form and generate vesicles which lose their coat and fuse with early endosomes, creating a late endosome. Extracellular components are delivered to the lysosomes for digestion and membrane components are recycled
Describe the process of receptor-mediated endocytosis
(Cell absorbs metabolites). Receptor is recognised by adaptor proteins and pits coated form. In the endosome it is released from its receptor (recycled) and/or delivered to the lysosome where it is released
Describe the process of phagocytosis
*specialised WBC (macrophages, neutrophils, etc) can take up microorganisms & dead cells.
*microorganisms bind to the surface of phagocytes and are recognised by cell surface receptors
*large vesicles (phagosomes) form which fuse with lysosomes so that contents can be digested
How is cell shape maintained?
By the cytoskeleton
Give 2 examples of when the cytoskeleton might need to be used
- Cell division
- Vesicle movement e.g. neurotransmitter release
Describe the structure of the cytoskeleton
- Filamentous structure, found in the cytoplasm & nucleus
- Formed of monomers assembled into repeat structures
- It is dynamic : assembles and disassembles to suit cells needs
- Is composed of 3 types of filaments : actin filaments; microtubules; intermediate filaments
Describe actin filaments
- smallest filaments
- made of globular protein actin assembled into 2 stranded helical polymers which line up to form bundles or 2D networks; or form 3D gels
- are dispersed through the cell
- roles in cell shape and motility
Describe microtubules
- made of globular protein tubulin (a & b subunits) which dimerise and form hollow units
- are more rigid: long and straight
- roles in positioning organelles, intracellular transport and mitosis
Describe intermediate filaments
- made of various intermediate filament proteins which are themselves filamentous
- extended as helical regions and wind together into dimers which associate into tetramers that wind together to form rope like fibres
- roles in mechanical support of cell structure
Explain the process of cytoskeleton assembly/disassembly
- monomers form end-to-end side-to-side interactions (non covalent = rapid assembly/disassembly as no bonds to break)
- rate of monomer addition relative to monomer concentration
- rate of disassembly is constant (net growth depends on monomer availability)
- at critical conc. of monomers, addition & removal at same rate = equilibrium phase
- accessory proteins can affect these processes - allowing cell control
- actin filements & microtubules have fast & slow growing ends
NUCLEOTIDE HYDROLYSIS - actin monomers carry an ATP molecule . Actin-ATP has high affinity for actin polymer (so is added)
- shortly after addition to the filament, ATP is hydrolysed to ADP
- actin-ADP has a lower affinity for the polymer (so is removed)
