Protein Sorting & Transport

• What are the two types of organelles in animal cells?


  Membraneless and membrane-enclosed organelles
• How do membraneless organelles function?

  They assemble when needed via non-covalent interactions
• What is the role of membrane-enclosed organelles?

  They form intracellular compartments with different functions
• What are the main features and functions of intracellular compartments or organelles?

  • Unique set of proteins in each compartment
  • Proteins synthesized in the cytosol
  • Selectively transported to needed locations
• What are the three mechanisms for transporting hydrophilic proteins across membranes?

  Transport mechanisms across impermeable membranes
• What is the function of signal sequences in proteins?

  They direct proteins to their destination

  
• Where are signal sequences commonly located in proteins?

  At the N-terminus

  
• What happens to proteins lacking a signal sequence?

  They remain in the cytosol
• How does deletion or addition of a signal sequence affect a protein?

  It alters the protein’s destination
• What is the structure of nuclear pores?

  They contain a tangled meshwork allowing selective transport
• What do nuclear pores allow to diffuse freely?

  Small, water-soluble molecules
• What must be displayed for selective transport of larger molecules through nuclear pores?

  A nuclear localization signal or export signal

  
• What is the outer membrane of the nuclear envelope continuous with?

  The ER membrane
• How many different proteins compose the nuclear pore complex?

  30 different proteins
• What do nuclear import receptors (NIR) recognize?

  Signal sequences for nuclear transport

  
• How are proteins and ribosomal components transported through nuclear pores?

  In their fully folded conformation or as assembled particles

  
• What drives the energy requirement for selective transport through nuclear pores?

  GTP hydrolysis mediated by Ran
• What happens when cargo-bound NIR interacts with cytosolic fibrils on the nuclear pore?

  It opens a passageway for transport

  
• What occurs to the cargo once NIR enters the nucleus?

  The cargo is released
• How are cytosolic proteins delivered to mitochondria and peroxisomes?

  By specific signal sequences recognized by receptors

  
• How are proteins destined for mitochondria translocated?

  Across both inner and outer membranes in unfolded form
• How are proteins destined for peroxisomes transported?

  In folded form by cytosolic receptor proteins
• What aids protein translocation across membranes?

  Chaperone proteins driven by ATP hydrolysis
• Where are proteins destined for the endomembrane system first transported?

  To the ER lumen

  
• What happens to proteins with an ER signal sequence?

  They are directed to the rough ER

  
• What type of proteins do free ribosomes synthesize?

  Proteins that remain in the cytosol

  
• What slows down translation when a ribosome synthesizes a polypeptide with an ER signal sequence?

  Binding of the cytosolic protein SRP

  
• What happens after SRP binds to the SRP receptor on the ER membrane?

  SRP is released and translation resumes

  
• What keeps the protein translocator open during translocation?

  ER signal sequence

  
• What happens to the ER signal sequence in soluble proteins?

  It is cleaved by a transmembrane signal peptidase

  
• What is the orientation of single-pass transmembrane proteins during translocation?

  1. terminus in ER lumen, C-terminus in cytosol

  
• What is vesicular transport in the endomembrane system?


  • Delivers soluble cargo proteins
  • Maintains membrane orientation
  • Involves secretory and endocytic pathways

  
• What do coated vesicles facilitate?

  Shaping of membrane and capturing cargo

  
• What type of vesicles are involved in outward secretory pathways?


  Clathrin-coated vesicles

  
• What role does adaptin 2 play in clathrin-coated vesicle budding?

  It traps cargo-bound receptors and secures clathrin

  
• How do cargo receptors recognize specific transport signals?

  By binding to specific signal sequences
• What are coated vesicles?

  Vesicles with a protein coat on cytosolic surface

  
• What is the function of coat proteins in vesicles?

  To shape membrane and capture cargo for transport

  
• How do combinations of coat proteins affect vesicles?

  They determine origin, destination, and cargo molecules

  
• What type of pathways are clathrin-coated vesicles involved in?

  Outward secretory and inward endocytic pathways