Intracellular Compartments and Protein Transport

Intracellular Compartments and Protein Transport

Overview of Chapter 15

This chapter explores the organization of eukaryotic cells into membrane-enclosed organelles and the mechanisms by which proteins are sorted and transported to their correct destinations. Key topics include organelle structure, protein sorting signals, and the pathways for protein transport within the cell.

• Membrane-enclosed organelles

• Protein sorting

• Intracellular transport

• Secretory pathways

• Endocytic pathways

Compartmentalization and Chemical Reactions in Cells

Why Do Cells Have Membrane-Enclosed Organelles?

Eukaryotic cells perform thousands of biochemical reactions simultaneously, many of which are incompatible. Compartmentalization allows cells to isolate and regulate these reactions efficiently.

• Biomolecular condensates (membraneless organelles): Regions where specific proteins and nucleic acids are concentrated by noncovalent interactions (e.g., nucleolus for ribosome assembly).

• Membrane-enclosed organelles: Structures surrounded by lipid bilayers, providing distinct environments for specialized functions.

Example: The nucleolus is a biomolecular condensate that concentrates proteins and RNA for ribosome assembly.

Advantages of Membrane-Enclosed Organelles

• Increase the surface area for reactions (e.g., inner mitochondrial membrane).

• Allow for the separation of incompatible metabolic processes.

• Enable the cell to maintain different environments (pH, ion concentrations) in different compartments.

Example: Lysosomes maintain an acidic environment for degradation, while the cytosol remains neutral.

Types of Organelles in Eukaryotic Cells

Major Membrane-Enclosed Organelles

• Nucleus: Contains the genetic material; surrounded by a double membrane (nuclear envelope).

• Endoplasmic Reticulum (ER): Site of lipid and protein synthesis; subdivided into rough (RER) and smooth (SER) regions.

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles.

• Lysosomes: Contain digestive enzymes for intracellular degradation.

• Peroxisomes: Carry out oxidation reactions and detoxification.

• Mitochondria: Generate ATP by oxidative phosphorylation; contain their own DNA.

• Chloroplasts (in plants): Site of photosynthesis; also contain their own DNA.

• Vacuoles (in plants and fungi): Storage and maintenance of cell turgor.

Endomembrane System

The endomembrane system includes the nucleus, ER, Golgi apparatus, lysosomes, peroxisomes, and plasma membrane. These organelles communicate via vesicular transport.

Semi-Autonomous Organelles

• Mitochondria and chloroplasts are considered semi-autonomous because they contain their own DNA and replicate independently, but still rely on the cell for most of their proteins.

• They are believed to have evolved from ancient symbiotic bacteria (endosymbiotic theory).

Protein Sorting and Transport

Protein Synthesis and Sorting

Most proteins are synthesized in the cytosol by ribosomes. Their final destination is determined by specific amino acid sequences called signal sequences.

• Free ribosomes: Synthesize proteins that remain in the cytosol or are imported into the nucleus, mitochondria, chloroplasts, or peroxisomes.

• Bound ribosomes: Attached to the ER; synthesize proteins destined for the endomembrane system or secretion.

Signal Sequences

Signal sequences are short stretches of amino acids (typically 15–60 residues) that direct proteins to their correct cellular location. Altering or removing the signal sequence can mislocalize the protein.

• Each organelle has characteristic signal sequences.

• Properties such as hydrophobicity and charge are important for recognition.

Destination	Example Signal Sequence
ER import	HN-Met-Met-Ser-Phe-Val-Ser-Leu-Leu-Leu-Ile-Leu-Phe-Trp-Ala-Thr-Glu-Ala-Glu-Gln-Leu-Lys-Cys-Glu-Val-Phe-Gln-
ER retention	-Lys-Asp-Glu-Leu-COO-
Mitochondria import	HN-Met-Leu-Ser-Leu-Arg-Gln-Ser-Ile-Arg-Phe-Lys-Pro-Ala-Thr-Arg-Thr-Leu-Cys-Ser-Ser-Arg-Leu-Leu-
Nucleus import	Pro-Pro-Lys-Lys-Lys-Arg-Lys-Val-
Peroxisome import	-Ser-Lys-Leu-

Additional info: Positively charged amino acids are often shown in red, negatively charged in blue, and hydrophobic residues in green in diagrams.

Protein Transport Pathways

• Gated transport: Proteins move between the cytosol and nucleus through nuclear pore complexes.

• Transmembrane transport: Proteins are transported across organelle membranes (e.g., mitochondria, ER, peroxisomes) via translocators.

• Vesicular transport: Proteins move between organelles of the endomembrane system via vesicles.

Protein Transport into the Nucleus

Nuclear Envelope and Pore Complexes

The nuclear envelope consists of two lipid bilayers (inner and outer membranes) and is continuous with the ER. Nuclear pore complexes (NPCs) are large protein assemblies that regulate the movement of molecules between the nucleus and cytosol.

• Small, water-soluble molecules can diffuse freely through NPCs.

• Larger proteins require a nuclear localization signal (NLS) for active transport.

Nuclear Localization Signal (NLS)

• Typically contains several positively charged amino acids (e.g., lysine, arginine).

• Recognized by nuclear import receptors (importins).

Mechanism of Nuclear Import

1. Nuclear import receptor binds to the NLS on the cargo protein in the cytosol.

2. The receptor-cargo complex interacts with cytosolic fibrils of the NPC and is guided through the pore.

3. Inside the nucleus, Ran-GTP binds to the import receptor, causing it to release the cargo protein.

4. The import receptor, now bound to Ran-GTP, returns to the cytosol.

5. In the cytosol, Ran-GAP stimulates GTP hydrolysis, converting Ran-GTP to Ran-GDP and releasing the import receptor for another round of transport.

Key Proteins in Nuclear Transport

• Ran-GTP: High concentration in the nucleus; promotes cargo release from import receptor.

• Ran-GDP: High concentration in the cytosol; import receptor is free to bind new cargo.

• Ran-GAP: GTPase-activating protein, located in the cytosol.

• Ran-GEF: Guanine nucleotide exchange factor, located in the nucleus.

Equation:

$\text{Ran-GTP} \xrightarrow{\text{Ran-GAP}} \text{Ran-GDP} + \text{P}_i$

$\text{Ran-GDP} + \text{GTP} \xrightarrow{\text{Ran-GEF}} \text{Ran-GTP} + \text{GDP}$

Example: Proteins destined for the nucleus retain their NLS even after import, allowing them to re-enter the nucleus after mitosis when the nuclear envelope reforms.

Summary Table: Protein Transport Pathways

Pathway	Destination	Mechanism
Gated transport	Nucleus	Through nuclear pore complexes
Transmembrane transport	ER, mitochondria, chloroplasts, peroxisomes	Via protein translocators
Vesicular transport	Golgi, lysosomes, plasma membrane, endosomes	Via membrane-bound vesicles

Additional info: The notes above are based on the provided materials and expanded with standard cell biology context for clarity and completeness.