Chapter 10:Protein Trafficking Through the Cytosol

Interactive Cellular Biology

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Chapter 10: Protein Trafficking Through the Cytosol

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Overview of Proteins in Cells

Abundance and Functionality

• Proteins are ubiquitous in the cell and can also be secreted from the cell.

• Mammalian cells can contain approximately 10,000 different types of proteins.

• Functions of proteins are diverse, occurring in various locations including:

  • Nucleus

  • Cytosol

  • Organelles

  • Extracellular Space

Importance of Protein Sorting

Questions Addressed

• If proteins are synthesized in the cytosol, why and how do they end up in other compartments?

  • Proteins can be found associated with membranes (integral and peripheral) inside organelles.

  • Some proteins are secreted to the outside of the cell.

Major Protein-Sorting Pathways in Eukaryotes

Key Components and Pathways

1. mRNA

2. Ribosomes

3. Rough Endoplasmic Reticulum (RER)

4. Signal-based targeting

5. Nuclear Transport (Nucleus)

6. Endoplasmic Reticulum Signal Sequence

7. Golgi Apparatus

8. Vesicle-based Trafficking

9. Secretory Pathway

10. Lysosomes

Protein Targeting and Addressing

Why are Proteins Targeted?

1. Proteins directed to specific organelles (e.g., ER, mitochondria, chloroplasts, peroxisomes, and nucleus) possess organelle-specific uptake or targeting labels.

2. Proteins destined for lysosomes or those that remain in the ER and Golgi enter the RER, traveling via vesicles, and contain specific address labels and retention sequences.

3. Proteins lacking targeting sequences remain in the cytosol.

Trafficking Routes Overview

• All proteins are synthesized by ribosomes in the cytosol.

• This leads to an initial bifurcation in pathways:

  • Cytosolic Trafficking: Destined for cytosol, nucleus, mitochondria, chloroplasts, or peroxisomes.

  • Endomembrane Pathway: Involves compartments and trafficking.

Protein Sorting Mechanisms

Selective Transfer

• Each compartment contains a distinct set of proteins that must be transferred selectively from the cytosol to the appropriate compartment.

• Differentiation must be made whether polypeptides are integral membrane proteins or lumenal proteins.

Understanding Lumen

Lumen:

• The inside space of a tubular structure, such as arteries or intestines, or cellular components like the endomembrane system.

The Endomembrane System

Pathway Overview

• Begins at the Endoplasmic Reticulum (ER), passing through the Golgi Apparatus.

• Potential end-destinations include:

  • Endosomes

  • Lysosomes

  • Peroxisomes

  • Cell Membrane

• Trafficking vesicles facilitate movement between organelles and membranes.

Signal Sequences in Proteins

Significance

• Signal sequences are encoded within the amino acid sequence of a protein and typically reside at the N-terminus (first exposed during translation).

• The drive behind targeting is influenced by:

  • Hydrophobicity: For targeting to the ER.

  • Positive Charge: For targeting to the nucleus.

• The signal sequence guides the protein to its final organelle destination.

Examples of Signal Sequences

• Notably, positively charged amino acids are shown in red, negatively charged ones in blue, and significant hydrophobic amino acids in green.

• Note: +H3N indicates the N-terminus; COO- indicates the C-terminus.

Trafficking to Mitochondria and Chloroplasts

Proteins from Different Genetic Origins

• Original genes of mitochondrial and chloroplast origins arose from prokaryotes.

• Most of these genes evolved to be part of the nuclear DNA.

• Distinction between:

  • Organelle-derived genes: Transcribed and translated within the organelles.

  • Nuclear-derived genes: Synthesized in the cytosol and translocated across organelle membranes.

Structure of Organelles

• Both mitochondria and chloroplasts consist of two peripheral membranes.

• Chloroplasts contain a third inner thylakoid membrane.

• Proper targeting is essential for import, requiring mitochondrial or chloroplast targeting signals.

Mechanism of Protein Import into Mitochondria and Chloroplasts

Key Characteristics of Targeting

1. Proteins that are encoded by nuclear genes and destined for mitochondria or chloroplasts are synthesized on cytosolic ribosomes and kept in an unfolded state by chaperones.

2. The N-terminal targeting sequences direct post-translational transport of these unfolded proteins through translocons (translocators) into the respective organelles.

3. Additional internal targeting sequences facilitate further targeting within the organelles towards various membrane and lumen destinations.

Steps in Trafficking to Mitochondria

1. Chaperones maintain proteins in an unfolded state at translation's start.

2. The mitochondrial targeting sequence directs the protein to the translocase of the outer mitochondrial membrane (TOM) at the end of translation.

Further Trafficking Steps

• Outer Mitochondrial Membrane Proteins: Inserted into the TOM, released into the membrane.

• Intermembrane Space Proteins: Pass through TOM.

• Inner Membrane Proteins: Pass through TOM and the translocases of the inner mitochondrial membrane (TIM), released into the membrane.

• Matrix Proteins: Pass through both TOM and TIM, then fold within the matrix.

Trafficking to Chloroplasts

Process Similarities

• Similar targeting processes as mitochondria using TOC and TIC instead of TOM and TIM.

• TOC (Translocon at the Outer Chloroplast Membrane): Transfers proteins to the intermembrane space.

• TIC (Translocon at the Inner Chloroplast Membrane): Transports proteins into the stroma.

This illustrates the journey from cytosol to thylakoid lumen including chaperone interactions.

Targeting of Peroxisomal Proteins

Import Mechanisms

• Luminal peroxisomal proteins are synthesized on free cytosolic ribosomes, containing specific targeting sequences.

• Their import occurs post-translationally via a cytosolic receptor protein and translocation machinery present on the peroxisomal membrane.

• Peroxisomal membrane proteins have different targeting sequences compared to matrix proteins, necessitating separate import pathways.

Trafficking to Peroxisomes

• Peroxisomes utilize both a cytosolic pathway for proteins, requiring a peroxisomal targeting signal (PTS), which translocates folded proteins, and an endomembrane pathway for proteins from the ER via Golgi vesicle fusion.

• Functions of Peroxisomes:

  • Detoxification

  • Lipid metabolism

Clinical Relevance

• Disorders in peroxisomal protein import can lead to Zellweger syndrome, characterized by severe abnormalities in brain, liver, and kidney function. [DOI link]

Transport Into and Out of the Nucleus

Mechanisms of Transport

• Large molecules (over 40 kDa) require nuclear-localization or nuclear-export signals, along with nuclear transport receptors, Ran G-proteins, and localized Ran-GEFs and GAPs.

• Other molecules, such as mRNPs, use a Ran-independent transport pathway.

Nuclear Components and Structures

• Nucleus Structure:

  • Nuclear Envelope: Comprises inner & outer membranes and nuclear pores.

  • Nucleoplasm: Contains nucleotides, enzymes, and liquid.

  • Nuclear Matrix: Composed of ribonucleoproteins, lamins, and attaches to the nuclear lamina.

  • Nucleolus: Site for ribosome synthesis.

  • Chromatin & Chromosomes: Includes DNA, RNA, and proteins affecting replication, transcription, and ribosome assembly.

Nuclear Pore Complexes (NPCs)

Functionality

• NPCs facilitate protein entry and exit through large, complex structures that are not merely open channels but supramolecular complexes—much larger than ribosomes—allowing specific transport of proteins, mRNAs, and ribosomal subunits.

Composition

• The nuclear pore structure includes cytosolic filaments, a central channel, and various rings ensuring regulation and selective permeability in nuclear transport.

Nucleocytoplasmic Transport Mechanisms

Types of Transport

• Diffusion (Passive): This mechanism applies to small molecules, ions, and small proteins.

• Signal-Based Transport (Active): Involves medium to large proteins and nucleic acids, which necessitate nuclear localization (NLS) and export signals (NES) for passage through NPCs.

Energy Requirements for Nuclear Transport

• Active nuclear transport relies on GTP hydrolysis, with GTP binding proteins acting as molecular switches.

• Key components include GTP (guanosine triphosphate) and GDP (guanosine diphosphate).

Summary of Active Nuclear Transport

Process Overview

1. GTP hydrolysis and special receptors are essential for cargo delivery to the nucleus.

2. Cargo possessing a nuclear localization signal (NLS) is recognized by importins which escort it through the NPC.

3. Once inside, Ran-GTP promotes release of cargo from the importin, completing the import process.

Active nuclear export follows similar steps using exportins and cargo with nuclear export signals (NES).

The intricate processes of protein trafficking, including into and out of nuclei, highlight the complex cellular machinery required for maintaining cellular compartmentalization and function.

Contrast of Protein Targeting Mechanisms

• Proteins are directed to their final locations by various mechanisms, including:

  • Signal-based targeting: Utilizes specific signal sequences in the protein to guide them.

  • Selective transfer: Each organelle has a specific set of proteins that must be selectively transported from the cytosol, distinguishing between integral membrane proteins and lumenal proteins.

  • Vesicle-based trafficking: Involves membrane-bound vesicles that transport proteins between compartments of the endomembrane system.

Comparison of Conformations During Transport

• During transport, proteins can adopt different conformations based on their localization:

  • Unfolded Conformation: Proteins destined for organelles like mitochondria and chloroplasts are maintained in an unfolded state by chaperones to facilitate translocation through translocons during import.

  • Folded Conformation: Many cytosolic proteins and those entering the endomembrane system retain their native folded structures, as they reach their destinations without needing to unfold.

Hierarchical Nature of Targeting Signals

• Targeting signals follow a hierarchical structure that dictates the final destination of proteins:

  • If a protein has both a nuclear localization sequence (NLS) and an N-terminal ER targeting sequence, it will first be directed to the ER. Proteins typically default to the organelle signal that takes precedence in targeting pathways.

Relationship Between the ER and Nuclear Membrane

• The Endoplasmic Reticulum (ER) is connected to the nuclear envelope, which consists of two membranes (inner and outer). This specific structure allows for direct communication and transfer of materials between the nucleus and the ER.

Organelles in the Endomembrane System

• The endomembrane system consists of:

  • Nuclear Envelope

  • Endoplasmic Reticulum (ER)

  • Golgi Apparatus

  • Lysosomes

  • Endosomes

  • Vesicles

  • Plasma Membrane

• The communication occurs through vesicular transport, where vesicles bud off one organelle and fuse with others, maintaining the flow of proteins and lipids in and out of the cell.

Fate of Proteins Lacking a Sorting Signal

• Proteins that lack targeting or sorting signals generally remain in the cytosol where they are synthesized. They do not enter organelles and are instead subjected to cytosolic degradation processes if they are not needed.

Nuclear Receptors and Transport

• Nuclear transport receptors (importins and exportins) facilitate the transport of proteins into and out of the nucleus:

  • Importins recognize and bind to proteins with nuclear localization signals (NLS), escorting them through the nuclear pore complexes (NPC).

  • Exportins transport proteins with nuclear export signals (NES) out of the nucleus, also utilizing the NPC for transport.

Role of NLS and NES in NPC Passage

• The nuclear localization signal (NLS) determines which proteins can be imported into the nucleus, while the nuclear export signal (NES) identifies proteins for export:

  • Proteins with the appropriate NLS are recognized by importins for transport into the nucleus, whereas proteins with NES are tagged for export by exportins.

Import of Mitochondrial and Chloroplast Gene Products

• Mitochondrial and chloroplast proteins must be imported into their respective organelles due to their origins in prokaryotic ancestors:

  • Most mitochondrial and chloroplast genes are now part of the nuclear genome and synthesized in the cytosol.

  • Import is facilitated through specific targeting signals in the protein structure; proteins are kept in an unfolded state by chaperones during the translocation into these organelles using the TOM and TIM for mitochondria, and TOC and TIC for chloroplasts.