Topic 8

• Protein sorting


  The process by which newly synthesized proteins are directed to their proper destinations within the cell

  
• Sorting signals


  Amino acid sequences that direct the delivery of proteins to locations outside the cytosol or the destination organelle

  
• Sorting receptors


  Proteins that recognize sorting signals and guide cargo into the appropriate compartment
• Transport of newly synthesized proteins to the nucleus

  1. Gated transport

  2. Protein translocation

  
• Gated transport


  The movement of proteins between the cytosol and the nucleus through nuclear pore complexes

  
• Nuclear localization signals


  Amino acid sequences that 'tag' a protein for import to the nucleus

  
• Nuclear export signals


  Amino acid sequences that 'tag' a protein for export from the nucleus to the cytoplasm

  
• Nuclear pore complexes (NPCs)


  • Large and elaborate structures that perforate the nuclear envelope in all eukaryotes
  • Composed of approximately 30 different proteins or nucleoporins

  
• Nucleoporins


  Nuclear pore transport factors that form stable channels through the nuclear envelope and control the bidirectional trafficking of molecules traversing the nuclear pores
• Importins


  Nuclear transport receptors that recognize nuclear localization signals and carry proteins through the nuclear pore complex into the nucleus

  
• Exportins


  Nuclear transport receptors that recognize nuclear export signals and mediate the export of proteins from the nucleus

  
• Translocation of newly synthesized proteins into the mitochondria

  Protein translocation

  
• Protein translocation


  The transport of specific proteins from the cytosol across a membrane (e.g. into the mitochondria, chloroplasts or peroxisomes) using transmembrane protein translocators

  
• One or more signal sequences direct all mitochondrial precursor proteins to their appropriate mitochondrial subcompartment

  
• Transmembrane protein translocators


  Proteins that transport specific proteins from the cytosol across a membrane (for example into the mitochondria, chloroplasts or peroxisomes)

  
• Signal sequences


  • Direct all mitochondrial precursor proteins to their appropriate mitochondrial subcompartment

  
• Signal sequence for mitochondrial protein import


  The first 18 amino acids of the precursor to a subunit of a protein that serves as a signal sequence for the import of the subunit into the mitochondrion

  
• Signal sequences that direct proteins into the matrix space

  • Always have the potential to form an amphiphilic α helix, which is recognized by specific receptor proteins on the mitochondrial surface

  
• Translocators


  Membrane protein complexes that mediate the import and sorting of mitochondrial proteins (as well as chloroplast and peroxisome proteins)

  
• TOM complex


  The translocator of the outer mitochondrial membrane that serves as the general entry gate for mitochondrial proteins and mediates the translocation of precursor proteins

  
• TIM complex


  Further translocates preproteins into the inner membrane and across the inner membrane into the mitochondrial matrix

  
• In animal cells, subtle variations exist in the subunit composition of the translocator complexes to adapt the mitochondrial import machinery to the particular needs of specialized cell types

  
• A protein could reach the mitochondrial matrix space by either crossing the two membranes all at once or crossing one at a time

  
• Mitochondrial proteins


  Synthesized on cytosolic ribosomes, unfolded during translocation, and transported as linear chains

  
• When preproteins cross the inner membrane and enter the mitochondrial matrix, molecular chaperones assist in their folding

  
• Energy sources that drive protein import in the mitochondria

  • ATP hydrolysis
  • Membrane potential across the inner mitochondrial membrane

  
• Proteins made on the ER ribosomes get to their destination (if they are not the ER proteins) using the third protein movement mechanism, called vesicular transport

  
• Vesicular transport


  Membrane-enclosed transport intermediates, mostly small spherical transport vesicles, ferry proteins from one compartment to another

  
• Cargo


  Membrane components and soluble luminal molecules carried by transport vesicles
• Secretory pathway


  Composed of membrane compartments specialized in protein folding, modifications, transport and sorting, involving the ER, Golgi apparatus, vesicles, and the cell membrane and lysosomes

  
• The role of the ER in protein processing and sorting was first demonstrated by George Palade and his colleagues in the 1960s

  
• Post-translational transport


  The process of transporting proteins to their final destination after they have been synthesized on ribosomes

  
• Co-translational transport


  Involves the growing nascent polypeptide chains being directly inserted into an ER translocation pore as they are synthesized on membrane-bound ribosomes

  
• Signal sequences in ER proteins


  Span about 15-40 amino acids, including a stretch of 7-12 hydrophobic residues, usually located at the amino terminus of the polypeptide chain as they emerge from the ribosome

  
• Free and membrane-bound ribosomes are functionally indistinguishable, and all protein synthesis initiates on ribosomes that are free in the cytosol

  
• Signal recognition particles (SRPs)

  Specific proteins that recognize and bind to the signal sequences as they emerge from the ribosome

  
• Signal sequences


  Span about 15–40 amino acids, including a stretch of 7–12 hydrophobic residues, usually located at the amino terminus of the polypeptide chain as they emerge from the ribosome

  
• Ribosomes that synthesize proteins for import into the ER are targeted to the ER by a signal sequence at their amino terminus of the growing polypeptide chain

  
• Signal recognition particles (SRPs)

  Specific proteins that recognize and bind to signal sequences as they emerge from the ribosome

  
• Mammalian signal recognition particle (SRP)

  • In the cytoplasm, SRP binds to ribosomes and signal sequences and inhibits further translation
  • SRP targets the entire complex to the rough ER by binding to the SRP receptor on the ER membrane