Organelle Movements and Vesicular Transport

Topographical Equivalence and Organelle Environments

• Organelle movements are biochemically distinct from the extracellular environment.
• Organelles are topographically equivalent to the extracellular environment.
• Topographical equivalence can be based on two factors:
    - Continuous spaces, such as nuclear space and cytosol.
    - Derivation from a common source.

Vesicular Transport

• Involves movement between topographically equivalent spaces.
• Vesicles originate from membrane regions that acquire specific protein markers.
• Protein markers serve two main functions:
    - Concentrate cargo molecules for transport.
    - Aid in the formation of the vesicle.
• Vesicle Formation Process:
    - Begins with a bulge on the membrane surface.
    - A protein arrangement forms a "basket structure" which is so named due to early microscopic observations.
    - The bulge grows as more proteins accumulate, eventually forming a spherical structure.
    - This structure is connected to the source membrane by a narrow neck.
    - The vesicle pinches off, becoming a self-contained unit comprising its cargo and associated proteins.

Protein Coats (Crates)

• Protein coats consist of different proteins that perform specific functions in vesicular transport.
• Functions of protein coats include:
    - Defining the vesicle's contents.
    - Assisting in the vesicle formation.
    - Directing the vesicle to its target location.
• Two main classes of protein coats:
    - Clathrin-coated vesicles
    - COPII-coated vesicles (the term "choraamerosis" is likely a misstatement in the transcript).

Clathrin-Coated Vesicles

Movement

• Clathrin-coated vesicles can move in both directions: inside-out and outside-in.
• Differences in movement are linked to subtypes:
    - Clathrin-coated type 1
    - Clathrin-coated type 2.

Structure and Assembly

• Clathrin-coated vesicles are assembled from several components, predominantly clathrin subunits.
• Clathrin subunits self-assemble into polyhedral cages which contribute to the vesicle formation.
• The self-assembly process is driven by the affinity of clathrin molecules and the energetically favorable conformation of the assembled structure compared to individual, dispersed molecules.
• This assembly can pull the underlying membrane with it during vesicle formation.

Key Components in Formation of Clathrin-Coated Vesicles

• Cargo Receptors:
    - Transmembrane proteins located in the ER membrane that bind to specific cargo molecules, initially inserted via co-translational events and possessing a conformational change upon cargo binding.
• Adaptin:
    - An intermediary protein linking cargo receptors to clathrin that attaches to the intracellular domain of cargo receptors after binding.
• Dynamin:
    - A protein responsible for pinching the vesicle off from the source membrane. It wraps around the neck of the forming vesicle and constricts it until the membrane pinches and fuses, forming a complete vesicle.

Mechanism of Clathrin-Coated Vesicle Formation

1. Cargo Binding:
      - Cargo molecules, such as acetylcholine and glutamate, bind to their specific receptors in the membrane.
2. Adaptin Recruitment:
      - Binding causes a conformational change in the cargo receptor which attracts adaptin.
3. Clathrin Assembly:
      - Adaptin also recruits clathrin, leading to further accumulation of clathrin molecules that self-associate to form a polyhedral cage, pulling the membrane inward.
4. Vesicle Budding:
      - The clathrin cage formation pulls the membrane into a budding vesicle. A narrow neck connects this budding vesicle to the source membrane at this critical point.
5. Dynamin-Mediated Pinching:
      - A chemical signal attracts dynamin to the neck of the budding vesicle. Dynamin constricts the neck, causing the vesicle to pinch off and fuse, separating it from the parent membrane.
6. Vesicle Maturation:
      - Once separated, dynamin and adaptin are released. The vesicle becomes "naked" or un-coated. The intracellular domain of the cargo molecules then directs the vesicle to its final destination. This mechanism is adaptable for various types of cargo and molecules.

Other Vesicle Coating Mechanisms

COPII-Coated Vesicles

• A different class of vesicles that utilizes a GTPase known as SAR1 (a "recruitment or ARF GTPase").
• Like other GTPases, SAR1 exists in two states:
    - GDP-bound form: Inactive.
    - GTP-bound form: Active, achieved by a guanine nucleotide exchange factor (GEF) facilitating GDP-GTP exchange.
• Hydrolysis of GTP to GDP, aided by a GTPase-activating protein (GAP), returns SAR1 to its inactive form.

SAR1 in the Cytosol vs. ER Membrane

• In its GDP-bound form, SAR1 is inactive in the cytosol.
• The GAP for SAR1 is transmembrane in the ER membrane, where SAR1 exchanges GDP for GTP, thus becoming active.

Mechanism of Vesicle Formation with SAR1

1. SAR1-GDP is in the cytosol.
2. It encounters GAP at the ER membrane.
3. GDP is exchanged for GTP.
4. The tail of GTP-SAR1 extends, inserting into the membrane.
5. More SAR1 molecules self-associate.
6. This forms a basket that evolves into a fully formed vesicle.
     - In this process, the "cargo" consists of ER membrane components, including lipids and proteins, destined for the plasma membrane to aid in cell growth in preparation for cell division.

RAB GTPases in Vesicle Traffic

• Another class of GTPases crucial for ensuring specificity between a vesicle and its target membrane.
• Like other GTPases, RAB GTPases cycle between:
    - RAB-GDP (inactive, cytosolic form).
    - RAB-GTP (active, membrane-bound form).
• Activation and inactivation mechanisms for RAB GTPases involve the action of GEFs (to exchange GDP for GTP) and GAPs (to hydrolyze GTP to GDP).
• The GTP-bound form of RAB interacts with a specific effector protein, facilitating precise docking with the target membrane.
• RAB GTPase involvement extends across both COPI and COPII vesicle formation and trafficking.
• Vesicles are transported by motor proteins along the cytoskeletal elements.
• The final stage of the RAB GTPase mechanism enables close proximity to the destination.

Role in Docking

• The active RAB GTPase, through its interaction with rab effectors, positions the vesicle membrane close enough to the target membrane for SNARE protein interaction.

Vesicle Fusion Mediated by SNARE Proteins

• SNARE proteins are vital for the fusion of vesicles with target membranes.
• There are two types of SNAREs:
    - t-SNAREs: Located on the target membrane.
    - v-SNAREs: Located on the vesicle membrane.
• Upon proximity facilitated by RAB GTPases, t-SNAREs and v-SNAREs wrap around each other spontaneously.
• This interaction is energetically favorable, releasing energy that must exceed the energy needed for membrane fusion, thus thermodynamically driving the fusion process.

Distinction between Docking and Fusion

• Docking: Initial attachment and positioning of the vesicle to the target membrane, ensured by specificity from RAB GTPases.
• Fusion: Actual merging of the vesicle membrane with the target membrane mediated by SNARE proteins.

Two Major Categories of Vesicular Traffic

Endocytic Pathway

• Involves bringing materials into the cell, starting with plasma membrane portions budding inward and enclosing external cargo.

Biosynthetic-Secretory Pathway (Exocytic Pathway)

• Involves moving materials out of the cell or to other organelles:
    - Can lead to the deposition of vesicle contents into the extracellular space.
    - Alternatively, vesicle membranes and associated proteins can integrate into the plasma membrane.
    - The term "exocytic" can imply various movements, including those occurring within the cell (to lysosomes or other organelles) and outward budding processes.

Overlap of Endocytic and Exocytic Pathways

• The endocytic pathway brings in molecules from the extracellular space, utilizing vesicles termed early endosomes.

• Simultaneously, the exocytic pathway transports digestive enzymes from the ER through the Golgi apparatus in vesicles.
    - These exocytic vesicles can fuse with endosomes, creating complex compartments that mix extracellular and intracellular materials.

• Early endosomes mature into late endosomes, which develop into lysosomes, facilitating digestion and transport of newly synthesized molecules (e.g., cholesterol).

Categories of Secretion

Regulated Pathway

• Materials are packaged into vesicles stored within the cell and remain quiescent until a specific signal triggers fusion with the plasma membrane for content release into the extracellular space.
    - Example: Neurotransmitter release from neurons triggered by calcium ion influx.

Constitutive Pathway

• Operates continuously without specific external signals, delivering materials essential for maintaining cellular functions:
    - Continuous secretion of newly synthesized proteins.
    - Transport of digestive enzymes to cellular locations.
    - Delivery of waste.
    - Incorporation of plasma membrane proteins and lipids into the plasma membrane.

The Golgi Apparatus: Structure and Function

Structure of the Golgi Apparatus

• Commonly depicted inaccurately as a simple stack of flattened sacs; the Golgi is actually an extensive network of flattened, membrane-bound sacs (cisternae).

Dynamic Nature of the Golgi Apparatus

• The Golgi is a dynamic, mobile entity:
    - Cisternae bud off from the ER and bong together to form larger stacks.
    - New cisternae form at the cis face (facing the ER), while older cisternae progress towards the trans face (facing the plasma membrane).
    - This movement supports material processing and modification.
• Vesicles bud off from the trans face to transport processed materials to their destinations, facilitated by motor proteins along microtubules.

Processing and Modification within the Golgi

• As materials traverse the Golgi cisternae, they are modified and prepared for their final functions.

Formation and Transport of Golgi Stacks

• Formation involves:
    - Vesicles budding from the ER travel towards the Golgi, fusing into a tubular cluster.
    - Proteins not part of the cargo are retrieved back to the ER through COPI vesicles.
    - The tubular cluster matures to form the cis-Golgi stack (cis face).

Lysosomes: Intracellular Digestion

Overview

• Lysosomes are responsible for intracellular digestion essential for every cell, differentiating from organismal digestion associated with external food breakdown.

Functionality

• Cells internalize molecular components through fusion with endocytic vesicles, leveraging enzymes within lysosomes for breakdown.
• Also contribute to breaking down cellular waste and debris.
    - In some cases, lysosomes assist in synthesizing new compounds from broken-down materials.

Lysosomal Enzyme Safety Mechanisms

• Lysosomal enzymes require a highly acidic environment (pH between 4.5-5) for optimal conformation and activity; the cytoplasm maintains a pH around 7.2.
• If lysosomal enzymes escape, they would denature due to neutral pH, prompting degradation by proteasomes.
• Maintaining this acidic environment necessitates ATP-driven proton pumps, making it energetically expensive.

Lysosome Formation and Maturation

• Hydrolytic enzymes arrive at lysosomes via Trans-Golgi Network vesicles.
• These vesicles bud from the Trans-Golgi and fuse with early endosomes, maturing these into lysosomes.

Pathways Involving Lysosomes

• Endocytosis: Materials taken into the cell are enclosed in vesicles that fuse with lysosomes for degradation.
• Phagocytosis: Engulfing large particles (e.g., bacteria) into phagosomes, which subsequently fuse with lysosomes.
    - Phagosomes form phagolysosomes, where enzymes are delivered for staged breakdown of material.
• Autophagy: The cell degrades its own components enclosed in autophagosomes that fuse with lysosomes for degradation.

Evidence for Endosymbiotic Theory

• The digestion process within lysosomes, reliant on various enzymes, possibly suggest an endosymbiotic origin for mitochondria and chloroplasts.
• It is hypothesized certain bacteria may develop symbiotic relationships with host cells to evade lysosomal degradation and increase host efficiency.

Specialized Endosomal Structures

• Phagocytosis: Engulfing external particles into phagosomes that fuse with lysosomes.
• Autophagocytosis: Involves engulfing intracellular components into autophagosomes that also fuse with lysosomes.
• The lecture notes conclude with a mention of specialized endosomal structures and their relevance in cellular processes, omitting further detail.