Organelles II: Protein Sorting and Membrane Trafficking

Organelles II: Protein Sorting and Membrane Trafficking

Protein Import Mechanisms for Membranous Organelles

  • Overview of organelles involved in protein import

    • Nucleus

    • Chloroplast

    • Mitochondrion

    • Peroxisome

    • Endoplasmic Reticulum (ER)

  • Proteins synthesized in the cytosol in relation to these organelles.

Protein Sorting

  • Distinction between types of ribosomes involved in protein synthesis:

    • Membrane-bound Ribosomes: Produce proteins destined for membranes, lysosomes, or secretion.

    • Free Ribosomes in Cytosol: Synthesize proteins that function in the cytosol.

  • Overview of mRNA structure in relation to protein synthesis:

    • mRNA structure depicted with 5' and 3' ends.

  • The destination of synthesized proteins:

    • Proteins can be directed to the Endoplasmic Reticulum lumen.

The Signal Theory

  • Introduced by Günter Blobel, who received the Nobel Prize in 1999.

  • Statement: “Proteins have three intrinsic signals that govern their transport and localization in the cell.”

  • Importance of intrinsic signals in determining the correct localization of proteins within the cell.

Soluble Compartment Protein Synthesis/Sorting

  • Focus on translocation and polypeptide synthesis.

  • Overview of the process:

    • Cytosol: Initial synthesis occurs here.

    • ER Lumen: Site of translocation for soluble proteins.

  • Diagrammatic representation involving the remaining polypeptide chain:

    • NH₂ (amino end) translocates through a channel into the lumen of the ER.

Integral Protein Synthesis/Sorting

  • Overview of the synthesis pathway for integral membrane proteins.

  • Key aspects of the synthesis process:

    • Integrated into the ER membrane.

    • Mature transmembrane proteins are formed in the ER membrane.

    • Focus on the signal sequence required for proper targeting.

  • Representation of the structure during the synthesis phase:

    • Detailing of NH₂ and COOH ends of polypeptides.

Protein Folding and Assistance

  • Role of Chaperonins (e.g., GroEL/GroES complexes) in protein folding.

  • Process of protein folding depicted:

    • Unfolded peptide enters the chaperonin complex.

    • ATP hydrolysis is involved in the folding process.

    • The resultant folded peptide exits the chaperonin structure.

Membrane Trafficking

  • Overview of the pathways through which proteins and other materials travel within and outside the cell.

  • Key organelles and structures involved in trafficking:

    • Nuclear envelope

    • Endoplasmic Reticulum

    • Lysosome

    • Golgi apparatus

    • Early and late endosomes

    • Transport vesicles

    • Plasma membrane

    • Extracellular space.

Post-Golgi Traffic

  • Description of the path taken by proteins post-Golgi apparatus.

  • Key steps in the process of vesicle trafficking:

    1. Selection of cargo molecules.

    2. Vesicle formation.

    3. Vesicle transport.

    4. Fusion with the target membrane.

Cargo Selection & Vesicle Formation

  • Deep dive into cargo selection processes involving:

    • Cargo receptors

    • Different transport vesicle coats:

    • COPII: Involved in ER to Golgi transport.

    • COPI: Part of the retrieval pathway from Golgi back to ER.

    • Clathrins: Involved in the formation of vesicles from the Golgi apparatus.

    • Dynamin: A GTPase involved in the final pinching off of vesicles.

Vesicle Transport & Targeting

  • Mechanisms for transporting vesicles:

    • Microtubules as tracks for vesicle movement.

  • Role of tethering factors and Rab proteins in targeting.

  • Conversion between GDP and GTP forms of Rab proteins as part of the targeting process.

Vesicle Fusion

  • Involves mechanisms such as SNAREs for the fusion of vesicles with target membranes.

  • Diagrammatic representation includes involvement of ions (e.g., Ca²⁺) in the process.

Golgi-Plasma Membrane Pathway

  • Steps in vesicle formation from the Golgi:

    • Involvement of adaptin and clathrin coat in cargo receptor interaction.

    • Uncoating process leading to naked transport vesicles.

    • Dynamin's role in the fission of vesicles.

Golgi to ER Retrieval Pathway

  • Detailed description of the COPI retrieval machinery:

    • Interaction of Arf1-GTP with cargo receptors for vesicle formation.

    • Recycling of Arf1-GDP back into the cytosol.

Summary of ER-Golgi Trafficking

  • Key steps summarized:

    1. Selection of cargo molecules.

    2. Vesicle formation using COP II, COP I, Clathrins, and Dynamin.

    3. Vesicle transport and targeting.

    4. Fusion with target membranes using SNARE proteins.

Summary of Post-Golgi Trafficking

  • Key outcomes of trafficking:

    • Outbound traffic including exocytosis and the continuous delivery of proteins and lipids to the plasma membrane.

    • Inbound traffic encompassing endocytosis pathways such as phagocytosis and pinocytosis.

Mitochondrial Functions

  • Focus on three main functions of mitochondria:

    1. Aerobic ATP production through the TCA cycle and electron transport chain.

    2. Heat production involved in thermogenesis.

    3. Control of specific metabolic pathways.

Mitochondrial Protein Import: Matrix Proteins

  • Involves the recognition of mitochondrial targeting sequences:

    1. Translocase of the Outer Membrane (TOM) complex recognizes the targeting sequence.

    2. Interaction with the Translocase of the Inner Membrane (TIM) complex; protein passes through the outer mitochondrial membrane (OMM).

Mitochondrial Protein Import: IMM Proteins

  • After reaching the inner mitochondrial membrane (IMM):

    1. TIM complex releases parts of the protein inside the IMM.

    2. Involvement of peptidases to remove targeting sequences.