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Lecture 3-10: Organelles and trafficking of proteins and vesicles

Organelles and Trafficking of Proteins & Vesicles

  • Key organelles involved in protein and vesicle trafficking:

    • Nucleus

    • Lysosomes

    • Peroxisome

    • Mitochondrion

    • Rough Endoplasmic Reticulum

Functions of Membrane-Enclosed Organelles

  • Cytosol: Houses metabolic pathways; facilitates protein synthesis.

  • Nucleus: Contains the main genome; involved in DNA and RNA synthesis.

  • Endoplasmic Reticulum (ER): Synthesizes most lipids; involved in protein synthesis for distribution.

  • Golgi Apparatus: Modifies, sorts, and packages proteins and lipids.

  • Lysosomes: Involved in intracellular degradation.

  • Endosomes: Sorts endocytosed material.

  • Mitochondria: ATP synthesis by oxidative phosphorylation.

  • Chloroplasts (in plant cells): ATP synthesis and carbon fixation via photosynthesis.

  • Peroxisomes: Oxidative breakdown of toxic molecules.

Modes of Transport for Proteins in Cells

  • Proteins can be transported to different cell compartments:

    • Nuclear Pores: Transport to the nucleus.

    • Translocators: Transport to organelles.

    • Vesicles: Transport proteins throughout the cell.

Protein Addressing via Amino Acid Sequences

  • Proteins have signal sequences determining their localization.

Typical Signal Sequences

  • Import into ER: -H3N-Met-Met-...

  • Retention in ER lumen: -Lys-Asp-Glu-Leu-COO-

  • Import into Mitochondria: -H₂N-Met-Leu-...

  • Import into Nucleus: -Pro-Pro-Lys-...

  • Export from Nucleus: -Met-Glu-Glu-Leu-...

  • Import into Peroxisomes: -Ser-Lys-Leu-

  • Colors indicate charge characteristics:

    • Red: Positively charged

    • Blue: Negatively charged

    • Green: Hydrophobic

Altering Protein Localization through Signal Sequences

  • Removal or alteration of signal sequences affects protein localization to organelles.

Nuclear Pores and Import Proteins

  • Nuclear Pores: Transport complexes of the nuclear envelope, allowing selective material entry.

  • Structure facilitates transport while maintaining a restrictive barrier.

Anatomy of the Nuclear Pore

  • Cytosolic fibrils enable selective protein binding without obstruction.

  • Disordered fibers act as diffusion barriers, controlling nucleus entry.

Nuclear Import Receptors

  • Import receptors recognize nuclear protein cargo in cytosol, facilitating entry through nuclear pores.

    • Once inside, cargo is released, and receptors return to cytosol.

  • Export proteins also travel similar pathways to transport mRNA out of the nucleus.

G Protein Ran in Nuclear Transport

  • Ran: Monomeric G protein regulating nuclear import receptor cycling.

    • Exists as Ran-GTP (active) or Ran-GDP (inactive).

    • Promotes cargo release from import receptors in nucleus.

Ran Cycle and Nuclear Import Process

  • Binding: Ran-GTP binding triggers import receptor to release cargo.

  • Recycling: Ran-GDP is formed in cytosol, allowing protein-import receptor return.

  • Other methods are involved in Ran return to nucleus.

Transporting Proteins to Mitochondria and Chloroplasts

  • Proteins intended for mitochondria/chloroplasts synthesized in the cytosol and imported via translocators uncovered during transport.

Peroxisome Protein Transport

  • Proteins targeted to peroxisomes are directly transported from the ER via transporters.

Endoplasmic Reticulum (ER) Overview

  • Largest organelle in terms of membranous area.

Translation of Nuclear Genome-Encoded Proteins

  • Ribosomes can be free or membrane-bound depending on the synthesizing protein's nature.

  • All nuclear mRNAs use the same ribosome pool for translation.

Role of ER Signal Sequences in Translation

  • Proteins with ER signals pause translation until ribosomes dock with the ER membrane.

Protein Transit Through the ER

  • Soluble ER proteins pass entirely through translocator; signal sequences are cleaved upon completion.

    • Remaining protein is folded and released into ER lumen.

Single-Pass Transmembrane Proteins

  • Translated with N-terminal signal; remaining portion either retained in membrane or synthesized in cytosol.

  • Results in a protein with distinct faces (luminal and cytosolic).

Multi-Pass Transmembrane Proteins

  • Contains internal signal sequences leading to multiple transmembrane domains via loops inserted into the ER.

  • Composed of alternating start/stop transfer signals determining transmembrane domain structure.

Vesicle Formation and Trafficking

  • Vesicles start and end at membranes; involved in exocytosis and endocytosis.

  • Essential for transporting molecules encapsulated in membranes around the cell.

Clathrin Coating in Vesicle Formation

  • Clathrin forms a basket structure that assists in vesicle budding.

    • Adaptins link clathrin to cargo receptors for selective vesicle formation.

Coated Vesicles Types and Functions

  • Different types of vesicles (clathrin-coated, COPII-coated, COPI-coated) are identified based on coat proteins, originating point, and destination.

Key Concepts Summary

  • Specific proteins are sorted to various cell regions via nuclear pores, translocators, and vesicles based on conserved amino acid sequences.

  • Synthetic addition of signal sequences can direct proteins effectively.

  • Nuclear import receptors ensure selective transport; G protein Ran facilitates the return process.

  • Different mechanisms exist for translocating proteins into mitochondria, chloroplasts, and ER, determining the final form based on localization and signal sequences.

  • Vesicle formation and composition vary according to their origins and targets, crucial for proper cellular function.