3. protein sorting

• Protein sorting involves the movement of proteins to their appropriate compartments within a cell
• Eukaryotic cells are compartmentalized, and each organelle requires specific proteins to perform their function
• The composition of proteins in each compartment is tailored to its particular needs to maintain cellular functions
• Gated transport refers to the selective movement of proteins between the cytoplasm and the nucleus through nuclear pore complexes in the nuclear membrane
• Transmembrane transport mechanisms facilitate the movement of proteins across the membranes of organelles such as the mitochondria and endoplasmic reticulum (ER)
• Vesicular transport involves the transport of proteins between membrane-bound compartments via membrane-bound vesicles
• A signal sequence is an amino acid sequence located at the terminal end of a protein that directs the protein to a specific destination within the cell
• Signal patches are composed of internal amino acid sequences within a protein that form a functional patch directing the protein to its appropriate destination
• Proteins such as polymerases and gene regulatory proteins are synthesized in the cytosol and need to be imported into the nucleus for functions like transcriptional regulation and DNA replication
• Certain molecules synthesized in the nucleus, like messenger RNA (mRNA) and ribosomal proteins, are exported to the cytosol for various cellular processes
• The nuclear envelope consists of inner and outer membranes separated by the perinuclear space, with nuclear pore complexes facilitating the transport of macromolecules
• The nuclear pore complex (NPC) consists of nucleoporins forming ring, scaffold, and channel proteins that regulate the passage of molecules between the cytosol and the nucleus
• The NPC allows the passage of molecules between the cytosol and the nucleus, with small molecules diffusing freely and larger proteins requiring active transport mechanisms
• Recent evidence suggests a tangled meshwork at the NPC enhances selectivity, allowing only specific molecules to pass through based on size and conformation
• Proteins transported through the NPC maintain their folded conformation, ensuring they are active upon reaching their destination within the nucleus or cytosol
• Nuclear import receptors recognize and bind to nuclear localization signals (NLSs) on cargo proteins, facilitating their selective import into the nucleus
• Import and export processes between the nucleus and cytosol require energy provided by the hydrolysis of GTP, with Ran acting as a molecular switch regulating nuclear transport
• The compartmentalization of Ran-GDP and Ran-GTP is crucial for the regulation of nuclear transport, with GTPase-activating protein (GAP) and guanine exchange factor (GEF) playing essential roles
• GAP and GEF play critical roles in maintaining the concentration gradient necessary for efficient nuclear transport
• GAP and GEF regulate the interconversion between Ran-GDP and Ran-GTP
• Compartmentalization ensures accurate and selective movement of proteins between the nucleus and cytosol
• Mitochondrial proteins are essential for functions like electron transport, oxidative phosphorylation, and ATP synthesis
• Most mitochondrial proteins are encoded in the nucleus and synthesized in the cytosol before being transported to the mitochondria
• Proteins destined for the mitochondrion can be transported directly to the matrix or inserted into the inner membrane
• Mitochondrial proteins are translocated in an unfolded state, crucial for threading the polypeptide through translocator complexes in the mitochondrial membranes
• The mitochondrial signal sequence is a specific sequence of amino acids located at one end of the protein
• The mitochondrial signal sequence is often structured as an alpha helix, providing stability and facilitating protein insertion into the mitochondrial membrane
• The TOM complex serves as the primary gateway for proteins destined for the mitochondria
• The TIM complex is responsible for transporting proteins across the inner mitochondrial membrane
• The OXA complex is involved in the insertion of proteins into the inner mitochondrial membrane
• Proteins are translocated across the outer mitochondrial membrane through the TOM complex
• Proteins are translocated across the inner mitochondrial membrane through the TIM complex
• The Cross-Bridge Ratchet Model describes the process by which mitochondrial Hsp70 actively pulls the polypeptide chain through the inner mitochondrial membrane
• The Cross-Bridge Ratchet Model ensures the efficient and accurate transport of proteins into the mitochondria
• Aspect: Insertion via OXA Complex

  • Protein is inserted into the inner membrane through OXA complex
  • OXA complex undergoes conformational change to release signal sequence
  • Signal sequence stabilized in membrane due to alpha-helical structure
  • Used for insertion of proteins produced within the mitochondrion
  • Involves interaction with OXA complex
• Aspect: Insertion via TIM23 Complex

  • TIM23 complex opens up and releases signal sequence into inner membrane
  • Signal sequence stabilized in membrane
  • Used for insertion of proteins into inner membrane
  • Simpler mechanism without involvement of additional complex