lecture 4- MEMBRANE DYNAMICS Flashcards
Learning Outcomes
*Explain the need for vesicular transport and describe the movement of vesicles between compartments within the cell.
*Describe how clathrin, as a coat protein, induces budding.
*Explain the importance of snare complexes and Rabproteins in vesicle fusion and docking with the target compartment.
*Apply principles of fusion and docking with membrane components to explain how organelles/compartment functions are maintained by vesicle transport.
*Discuss the synthesis of lipids within the ER and how these migrate to the cell surface
How do all the phospholipid components all link together
➢Components of the membrane determine the properties of the membrane.
➢Different cells have different functions.
➢Parts of the plasma membrane may have different functions.
➢Organelles have different functions
➢Therefore different cell types (or organelle types) must have different components in their membranes
Endocytosis and Exocytosis
Vesicle transport
- What membrane components change between ER and golgi? How does this influence the properties of the membrane?
The transition from the endoplasmic reticulum (ER) to the Golgi apparatus involves several changes in membrane components, which significantly influence the properties and functions of the membranes. Here are the key components that change and how these changes affect the membrane properties:
- Lipid Composition
Change:
Phospholipids: The types and proportions of phospholipids differ between the ER and Golgi membranes. The Golgi apparatus tends to have a higher concentration of certain sphingolipids and cholesterol compared to the ER.
Cholesterol: The Golgi membranes generally have a higher cholesterol content, contributing to membrane fluidity and stability.
Influence on Membrane Properties:
Fluidity: The increased cholesterol in the Golgi helps to stabilize the membrane structure and maintain fluidity at varying temperatures, which is crucial for the proper function of Golgi-resident proteins.
Curvature and Vesicle Formation: Specific lipid compositions influence membrane curvature, which is important for vesicle budding and fusion processes during transport to and from the Golgi.
- Protein Composition
Change:
Membrane Proteins: The types of integral and peripheral membrane proteins also differ between the ER and Golgi. The Golgi membrane contains specific enzymes (e.g., glycosyltransferases) that are essential for post-translational modifications, such as glycosylation.
Glycosylation: Glycoproteins synthesized in the ER undergo further modifications in the Golgi, leading to changes in their glycosylation patterns.
Influence on Membrane Properties:
Functional Diversity: The unique set of proteins in the Golgi allows it to perform specific functions, such as modifying, sorting, and packaging proteins for secretion or delivery to other organelles.
Recognition and Interaction: The presence of specific glycoproteins and glycolipids in the Golgi membrane plays a crucial role in cell-cell recognition and communication, influencing the interactions with other cells and extracellular components.
- Glycolipids and Glycoproteins
Change:
The Golgi apparatus is rich in glycosylation processes, leading to the synthesis of diverse glycolipids and glycoproteins that are not as prevalent in the ER.
Influence on Membrane Properties:
Cell Recognition: Glycolipids and glycoproteins are involved in cell signaling, adhesion, and recognition processes. This is crucial for immune responses and cell-cell interactions.
Stability and Protection: The presence of carbohydrate chains can provide stability to the membrane and protect against proteolytic enzymes. - Membrane Asymmetry
Change:
The asymmetrical distribution of lipids and proteins is more pronounced in the Golgi than in the ER. The Golgi membrane has distinct lipid and protein compositions on the cytoplasmic and luminal sides.
Influence on Membrane Properties:
Functionality: The asymmetry is crucial for the functionality of membrane proteins and for maintaining the distinct environments necessary for enzymatic activity, protein sorting, and signaling.
Vesicle Formation: The specific distribution of lipids and proteins influences the budding and fusion of vesicles, affecting intracellular transport. - Membrane Potential and Ionic Composition
Change:
The ionic composition and membrane potential may vary between the ER and Golgi, affecting how proteins and ions are handled as they pass through these organelles.
Influence on Membrane Properties:
Transport Mechanisms: Differences in ionic composition can influence the activity of transport proteins and channels, affecting how substances are moved in and out of the Golgi.
Summary
The changes in membrane components between the ER and Golgi—including lipid composition, protein diversity, glycosylation patterns, membrane asymmetry, and ionic composition—significantly influence the properties and functions of these membranes. These differences are crucial for the Golgi’s role in processing, modifying, and sorting proteins, ultimately impacting cellular communication, metabolism, and homeostasis. Understanding these changes is essential for grasping the complexity of intracellular transport and membrane dynamics.
Address labels
Membrane components act as address labels.
Glyco-additions such as mannose enable receptor binding and direct transport to specific compartments
Cellular compartments
Endocytosis is a way of sampling the environment.
Type of coat protein determines destination of the vesicle.
Directions of movement
Lipids and compartments: Summary
➢The contents of the membrane can be regulated by endocytosis and exocytosis.
➢Membrane components made in the ER contain low levels of cholesterol increasing as the membrane matures through the golgiand to the plasma membrane.
➢Membrane bound proteins generated in the ER can act as cargo receptors and/or are proteins destined for other compartments e.g. lysosome, plasma membrane etc.
➢Vesicular transport enables secretory proteins and membrane bond & associated proteins to be transported to the plasma membrane while also allowing retrograde transport into the cell of extracellular environment, membrane recycling and receptor recycling and degradation.
Structure of a Clathrincoats
To bend the membrane.
Only recruited to receptors associated with plasma membrane
Coat formation
Coat assembly & cargo selection: Proteins on the ER membrane act as receptors to keep cargo in a specific place. Binding leads to a confirmational change that enables adapter proteins to be recruited.
Bud formation:Adapter proteins (such as AP2) bind to clathrintriskelions. Bar-domain containing proteinsaid the bending of the membrane to increase access to the adapter proteins to clathrin.
Vesicle formation: Continuation of formation of a vesicle and generation of a stalk, which then recruits dynaminand accessory proteins to pinch off the vesicle.
Uncoating: After the vesicle is released there is no longer need for the coat proteins & associated adapters, so they are disassembled and recycled. This leaves the vesicle with cargo-bound receptors on the vesicle surface (and snare proteins to direct the vesicle to its destination)
Initiation of coat formation
Binding of μ2 and σ2 to PIP2is required to initiate cargo receptor binding
Change in receptor conformation leads to membrane deformation, making it more accessible for coat proteins
Coat proteins bend the membrane
Coat proteins play a crucial role in membrane dynamics, particularly in the process of vesicle formation. These proteins are essential for creating the membrane curvature necessary for budding and fission during vesicle transport. Here’s how coat proteins facilitate membrane bending:
- Binding to Membrane Lipids
Initial Interaction: Coat proteins, such as clathrin, COPI, and COPII, first bind to specific lipid molecules on the cytoplasmic side of the membrane. This binding can be facilitated by the presence of lipid modifications or specific lipid compositions that attract these proteins.
Conformational Changes: Upon binding, coat proteins undergo conformational changes that help them organize into a lattice-like structure. - Formation of Protein Lattices
Lattice Structure: For example, clathrin forms a triskelion shape, with three arms that extend outwards. When multiple clathrin triskelions come together, they create a basket-like structure around the membrane.
Mechanical Forces: The assembly of these coat proteins into a lattice generates mechanical forces that induce curvature in the lipid bilayer. The close packing of the proteins and their geometry pulls on the membrane, leading to bending. - Creating a Localized Domain
Concentration of Membrane Components: The assembly of coat proteins causes a localized increase in the concentration of membrane components, such as lipids and other proteins. This concentration can further promote curvature by altering the lipid bilayer’s physical properties.
Exclusion of Larger Lipids: The interaction of coat proteins with specific lipids can create regions of exclusion for larger lipids, which can contribute to the curvature by allowing the smaller lipids to dominate the local area. - Recruitment of Other Proteins
Adapter Proteins: Coat proteins can recruit other proteins (such as adaptor proteins) that facilitate further membrane bending or help with cargo selection. These adaptor proteins can also interact with both the coat proteins and the membrane, enhancing the curvature.
Other Structural Proteins: Proteins like dynamin can be recruited to the site of budding, which is crucial for the fission process. Dynamin wraps around the neck of the forming vesicle and constricts, facilitating the final pinching off of the vesicle from the membrane. - Dynamic Regulation
GTP Hydrolysis: In some cases, like with dynamin, the hydrolysis of GTP provides the energy necessary for conformational changes that further enhance membrane constriction and bending.
Coat Protein Disassembly: After vesicle formation, coat proteins are typically disassembled to allow the vesicle to mature and fuse with its target membrane. This disassembly also helps restore the original membrane curvature.
Summary
Coat proteins induce membrane bending primarily through their ability to bind to membrane lipids, form dynamic protein lattices, and create localized domains that alter the physical properties of the membrane. The mechanical forces generated during this process are essential for the formation of vesicles that transport proteins and lipids throughout the cell. Understanding the mechanisms by which coat proteins bend membranes is crucial for comprehending cellular trafficking, signaling, and membrane dynamics.
Coat proteins need friends to bend the membrane
✓Membrane-bending proteins that contain crescent-shaped BAR domains, these cause a shape change to the membrane via electrostatic interactions with the lipid head groups.
✓BAR-domain proteins are thought to help AP2 recruit clathrinby shaping the plasma membrane to allow a clathrin-coated bud to form.
✓Some proteins also contain amphiphilic helices that cause membrane bending when inserted into the cytoplasmic leaflet.
✓These are essential to shaping the neck of a budding vesicle through stabilization of sharp membrane bends.
✓Clathrinmachinery recruits local assembly of actin filaments that introduce tension to help pinch off and propel the forming vesicle away from the membrane
Vesicle release
Dynamin recruits a protein complex to initiate vesicle release.
Inner leaflets of the bilayer merge, then the vesicle “pinches off”.
Dynamin
The GTPase domain of dynamin changes shape with GTP hydrolysis.
Shape change likely results in the “squeezing” of the neck of the budding vesicle
Rab proteins and their role
Monomeric GTPases.
Each Rabprotein is associated with one or more organelle.
At least one type of Rabis present on the cytosolic side of each organellesmembrane
Rabfamily proteins & locations
Lipid droplets: Same story different cargo
Generation of cholesterol through the HMG CoA-reductase, resident in the ER.
Products are not amphipathic therefore need to be coated in membrane to enable transport through the aqueous cytosol
Linking things together: LDL uptake
Excess LDL can control cholesterol synthesis in the liver. This detection system requires many of the principles we have covered:
*Endocytosis of LDL particles using clathrincoated pits.
*Recycling of LDL receptors to the plasma membrane.
*Formulation of a vesicle containing cholesterol esters which is integrated into the ER.
*High cholesterol levels in the ER results in a decrease in the synthesis of HMG-CoA reductase and LDL receptors.
