1 intracellular compartments and protein transprort

• All eukaryotic cells have the same basic set of membrane-enclosed organelles
• The plasma membrane encloses the cytosol
• Evolution of membrane-enclosed organelles


  1. Invagination of the plasma membrane (nuclear membrane and ER)

  2. Endosymbiosis (mitochondria and plastids)
• Topologically equivalent spaces


  Cellular compartments between which molecules can traverse without having to cross membranes
• Signal sequences


  Sequences that direct proteins to the correct cellular address
• Sorting receptors


  Receptors that direct proteins to the correct cellular address
• Ways proteins can move between compartments


  • Gated transport
  • Transmembrane transport
  • Vesicular transport
• Gated transport - Transport of molecules between cytosol and nucleus


  1. Nuclear pores are gated

  2. Large molecules need to be chaperoned

  3. Importins mediate import

  4. Exportins mediate export
• Nuclear localisation signals


  Sequences that direct nuclear proteins to the nucleus
• Nuclear import receptor proteins (importins)


  Bind to nuclear localisation signals and nuclear pore complex proteins
• Nuclear export

  Works similarly to nuclear import, but in reverse
• GTPases


  Molecular switches or timers in cellular processes, exist in GTP-bound (on) or GDP-bound (off) forms
• GEF

  Guanine exchange factor
• GAP


  GTPase activating protein
• Ran GTPase


  Imposes directionality on transport through nuclear pore complexes, can be activated (Ran-GTP) or deactivated (Ran-GDP)
• Ran-GTP is preferentially bound by the nuclear import receptor in the nucleus


  This frees the imported NLS-containing protein
• Ran-GTP must be converted to Ran-GDP in the cytosol


  So the nuclear import receptor can start another cycle
• Ran-GDP then diffuses back into the nucleus where it is converted to Ran-GTP
• The nuclear pore complex has cytoplasmic filaments and nucleoporins that interact with the importin to transit the pore
• Regulation of transport through nuclear pores


  Controlling access to transport signals on proteins (e.g. NF-AT)
• Transmembrane transport - Transport of proteins into mitochondria and chloroplasts


  1. Proteins unfold to enter

  2. Depend on signal sequences and protein translocators

  3. Mitochondrial precursor proteins imported as unfolded polypeptides
• TOM complex


  Translocase of the Outer Membrane (mitochondria)
• TIM complex


  Translocase of the Inner Membrane (mitochondria)
• Transport into chloroplast thylakoid membrane


  1. Two signal sequences direct proteins

  2. TOC and TIC protein translocator complexes
• TOC


  Translocase of the Outer Chloroplast membrane
• TIC


  Translocase of the Inner Chloroplast membrane
• The ER is structurally and functionally diverse
• Transmembrane transport - Transport of proteins into the endoplasmic reticulum


  1. Proteins destined for many organelles are inserted into the ER as they are translated

  2. Co-translational insertion

  3. The ER lumen is continuous with the space between the inner and outer nuclear membranes
• Rough ER


  When proteins destined for secretion are being co-translationally inserted
• Signal-recognition particle (SRP)


  Directs the ER signal sequence to a specific receptor in the rough ER membrane
• Translocator/Translocon


  Pore complex in the ER membrane that proteins with ER signal sequences are directed to
• Sec61 complex


  The core of the ER protein translocator
• The polypeptide chain passes through an aqueous channel (Sec61 complex) in the translocator
• Sec61


  A heterotrimer formed from 3 different protein subunits (Sec ⍺, 𝛾, and β)
• Sec61 is tightly closed by a 'plug' domain when not engaged in co-translational insertion