Protein Transport Study Notes

Protein Transport

Introduction to Vesicular Transport

  • Concept: Vesicles bud from one organelle and fuse with another, transferring contents and facilitating intracellular transport.

  • Figure Reference: Figure 15-19 from ECB, 6th edition illustrates this process.

Vesicle Trafficking inside Cells

  • Function: Enables directed transport of:

    • Membrane proteins

    • Soluble proteins

    • Lipids

  • Tools for Trafficking: Each cellular membrane contains:

    • Specific Rab GTPases

    • Phosphoinositides (PIPs)

    • Coat proteins

    • SNAREs

  • Roles of Rabs and PIPs:

    • Contribute to membrane identity

    • Recruit regulators of membrane trafficking to the endomembrane surface

  • Vesicle Formation:

    • Specific v-SNAREs are incorporated during budding.

    • Fused with target membranes possessing complementary t-SNAREs.

Review: Vesicle Formation & Membrane Identity

  • RABs and PIPs: These molecules are crucial in differentiating membrane identity by transitioning between conformations as monomeric GTPases.

Selective Endocytosis: Clathrin-Mediated Endocytosis (CME)

  • Key Components:

    • Donor: The initial membrane where vesicle begins to bud (e.g., plasma membrane).

    • Adaptor Proteins: Facilitate the incorporation of cargo.

    • Membrane Proteins: Specific proteins that assist in the selection of cargo.

    • Cargo Molecules: Substances that need to be transported.

  • Process Flow:

    1. Coat Assembly: Involvement of clathrin, which forms a triskelion shape.

    2. Cargo Selection: Cargo receptors recognize and bind specific cargo molecules.

    3. Bud Formation: The vesicle begins to bud from the membrane.

    4. Uncoating: Clathrin coat must be removed to allow fusion with target membrane.

  • Final Transport: Results in the formation of a naked transport vesicle.

Receptor-Mediated Endocytosis: LDL

  • Fate of LDL and Receptor: They undergo different fates upon sorting, mediated by the geometrical configuration of the early endosome.

  • Impact of Research:

    • Brown and Goldstein utilized cell culture techniques to explore familial hypercholesterolemia (FH).

    • Genetic mutations in the LDL receptor were identified as significant factors.

    • This foundational research informed the development of statins, used therapeutically for cholesterol regulation.

Implications of Excess Cholesterol

  • Health Risks:

    • Excess cholesterol accumulates in arterial walls leading to plaque formation (atherosclerosis).

    • Plaques can obstruct blood flow, increasing risks for heart attacks or strokes.

  • Cholesterol Accumulation: A long-term process that can span decades.

Endosome Maturation Process

  • Complexity: The process of endosome maturation is intricate and involves several stages and structures.

  • Key Structures Involved:

    • Early endosomes

    • Recycling processes to the plasma membrane

    • Formation of multivesicular bodies

    • Fusion with lysosomes to form endolysosomes.

  • Figure Reference: Figure 13-50, MBOC, 7th edition.

Molecular Markers and Endosomal Compartments

  • Research by Elkin, Lakoduk, et al. (2016): Identified different Rab and PIP combinations as molecular markers for distinct endosomal compartments.

Summary of Multivesicular Bodies (MVBs)

  • Definition: Multivesicular bodies (MVBs) contain intralumenal vesicles (ILVs).

  • Process Overview:

    • Formation: Involves invagination and pinching off of membrane sections (sequestration).

    • Function: MVBs fuse with late endosomes or lysosomes to facilitate degradation processes.

  • Signaling Involvement: MVBs also play crucial roles in signaling by harboring receptors.

  • Figure Reference: Figure 13-59 from MBOC, 7th edition illustrates this process.

Lysosome Maturation Model

  • Pathway: Lysosome maturation involves several stages with the transformation of endosomal compartments into lysosomes.

  • Important Components:

    • Intralumenal vesicles

    • Late endosomes

    • Digestive enzymes within the lysosome.

  • Hydrolytic Function: Lysosomes are hydrolytically active and must operate in low pH environments to digest contents.

  • Figure Reference: Figure 13-59 from MBOC, 7th edition.

Characterization of Lysosomes

  • Size: Typically range between 0.2-0.5 μm.

  • pH Levels:

    • Cytosolic pH: 7.2

    • Lysosomal pH: 5.0

  • Functional Enzymes: Include:

    • Nucleases

    • Proteases

    • Glycosidases

    • Lipases

    • Phosphatases

    • Sulfatases

    • Phospholipases

  • Role of H+ Pump: Maintains acidic conditions essential for enzymatic activity, requiring ATP for function.

  • Figure Reference: Figure 15-36 from ECB, 6th edition.

Pathways to Degradation in Lysosomes

  • Mechanisms: Multiple pathways facilitate the degradation of cellular components via lysosomal activities:

    1. Endocytosis: Involves internalization of extracellular materials.

    2. Phagocytosis: Engulfs large particles such as bacteria.

    3. Autophagy: Targets organelles and larger aggregates within the cytoplasm for degradation.

  • Integration: These pathways highlight the lysosome's role in cellular recycling and quality control.

  • Figure Reference: Figure 15-37 from ECB, 6th edition.

Autophagy

  • Definition: A process by which organelles and larger cytoplasmic aggregates are degraded within lysosomes.

  • Figure Reference: Figure 14-34 from Lodish, 9th edition illustrates the process of autophagy.