Unit 04 Pt1

Unit Overview

  • Unit Title: Intracellular Trafficking & Coats

  • Course: BIOL 331 – Advanced Cell Biology

  • Source: Molecular Biology of the Cell, 6th Edition, Alberts et al., Garland Science; 2022

  • Page Range: 749 - 807

Part 1: Outline of Topics

  • Various Types of Coated Vesicles

  • Clathrin Coat Assembly and Vesicle Formation

  • Role of Adaptor Proteins in Clathrin-coated Vesicles

  • Function of Phosphoinositides in Membrane Domains

Intracellular Vesicular Traffic

  • Membrane System:

    • Eukaryotic cells are organized into a system of membranes to facilitate communication, nutrient capture, and response to environmental changes.

  • Exocytosis:

    • Delivers proteins, lipids, and carbohydrates to the plasma membrane or cell exterior via the biosynthetic secretory pathway.

  • Endocytosis:

    • Cells excise plasma membrane (PM) into endosomes for protein and nutrient capture, with subsequent delivery to lysosomes.

    • Endosomes can be recycled to PM or degraded in lysosomes.

Steps in Vesicle Transport

  • Transport Vesicles:

    • Continuously bud from one membrane and fuse with another, serving as cargo carriers for soluble and membrane proteins.

    • Remarkably selective regarding cargo and destination.

Membrane Transport: Defining Compartments

  • Molecular Markers:

    • Compartment membranes have specific markers guiding incoming vesicles; specificity comes from unique combinations of markers.

  • Segregation of Components:

    • Membrane patches enriched in specific components bud off and transfer to other compartments.

  • Protein Assembly into Domains:

    • Special protein coats assemble on cytosolic membrane surfaces, influencing vesicle specificity.

    • GFP fusion proteins help elucidate protein trafficking.

Coated Vesicles

  • Coated Regions:

    • Specialized membrane regions from which vesicles bud off, forming distinct coats.

    • Coat Layers:

      • Inner Layer: Concentrates membrane proteins and directs vesicle formation.

      • Outer Layer: Forms a basket-like structure that deforms the membrane patch into a vesicle shape.

Types of Coated Vesicles

  • Clathrin-Coated:

    • Transport from Golgi and PM.

  • COPI-Coated:

    • Transport from the Golgi.

  • COPII-Coated:

    • Transport from the ER.

Clathrin-Coated Vesicles

  • Key Component:

    • Clathrin formed from triskelion structures (three large and three small polypeptide chains).

  • Structure and Architecture:

    • Triskelions can spontaneously form polyhedral cages, dictating the geometry of the clathrin cage.

  • Formation of Pits and Vesicles:

    • Clathrin assembly creates coated pits on membranes, facilitating vesicle budding.

Adaptor Proteins in Coated Vesicles

  • Role of Adaptor Proteins:

    • Link clathrin coat to membrane; important for cargo specificity.

    • Trap transmembrane proteins with soluble proteins for efficient vesicle trafficking.

  • Adaptin Variability:

    • Specificity of adaptor proteins ensures different cargo is carried by clathrin-coated vesicles from various membranes.

    • Key adaptins involved in post-Golgi trafficking.

Phosphoinositides (PIPs)

  • Definition and Importance:

    • Inositol phospholipids are regulatory, comprising less than 10% of membrane phospholipids but critical based on phosphorylation state.

    • Types:

      • Unphosphorylated: PI (phosphatidylinositol)

      • Phosphorylated: PIPs (phosphoinositides)

  • Functionality:

    • Specific interconversion of PIs and PIPs is regulated, providing distinct domains for vesicular transport.

Conclusion and Questions

  • Experimental Approach:

    • Investigate protein trafficking via vesicular transport.

  • Coat Relevance:

    • Understand roles of triskelion-based coats and adaptors.

  • Pathways:

    • Recognize locations of COPII, COPI, and clathrin-coated vesicles in cellular function.