Exam 2

• Cytosolic side of plasma membrane

  Usually has negative charge so positive ions tend to be pulled into the cell

  
• Sodium (Na+)


  Low concentration inside cell, high concentration outside cell
• Sodium

  Has high electrochemical gradient because concentration and voltage work in same direction
• Potassium (K+)


  High concentration inside cell, low concentration outside cell
• Potassium


  Has low electrochemical gradient because concentration and voltage work in opposite directions
• Membrane Transport


  • Active transport
  • Coupled transporter
  • ATP-driven pumps
  • Light-driven pumps
• Na+- K+ pump


  ATP-driven pump that transports Na+ outside of cell, and brings K+ inside cell (against gradient)
• Na+- K+ pump


  1. Phosphorylation by ATP causes it to undergo several conformational changes for active transport

  2. Transports 3 sodium ions out, 2 potassium ions in

  3. Maintains ion concentrations for sodium and potassium

  4. Maintains negative charge inside cell
• Effects of Na+- K+ pump

  High electrochemical gradient of Na+ is used to transport other solutes across membrane
• Coupled transport


  Solute that travels down its gradient provides energy for different solute that travels against its gradient
• Glucose-Na+ symport pump


  1. Transports glucose into intestinal cells by using high electrochemical gradient of Na+

  2. Pump restricted to apical domain by tight junctions

  3. Glucose transported from the gut into the cell (against concentration gradient, active transport)

  
• Glucose transporters (uniports)


  Release glucose into bloodstream (with concentration gradient, passive transport)

  
• Three Critical Steps for Neuronal Signaling


  • Neuron (with a negative resting membrane potential) receives a depolarizing stimulus
  • Depolarizing stimulus exceeds threshold potential and activates voltage-gated Na+ channels (action potential)
  • Sodium channels open long enough to activate neighboring sodium channels (propagation); signal travels forward towards nerve terminal

  
• Excitatory Neurotransmitters


  • Acetylcholine and glutamate activate ion channels which transport positive ions (Na+)
  • Influx of positive ions pushes membrane potential towards threshold for action potential

  
• Inhibitory Neurotransmitters


  • GABA and glycine activate ion channels which transport Cl- ions
  • Influx of negative ions pushes membrane potential away from threshold for action potential
• Eukaryotic cells have many different membrane-enclosed organelles which provide intracellular compartments to perform a variety of chemical reactions
• Lipid bilayers on organelles provide selectively permeable barriers that allow the transport of specific molecules
• Each organelle contains a unique set of proteins which allows it to perform a specific function
• Protein Sorting


  • Free ribosomes: mitochondria, peroxisomes, and the interior of the nucleus receive proteins form the cytosol
  • Membrane-bound ribosomes: Golgi apparatus, lysosomes, and plasma membrane receive proteins from ER
• Sorting signal


  Amino acid sequence in protein used to direct movement of protein inside cell
• Different organelles use different signal sequences
• Signal sequence


  Usually attached to N-terminus and removed once sorting process is complete
• Signal patch


  Three-dimensional arrangement of amino acids that can also act as a sorting signal
• Proteins that do not have signal sequence or signal patch are cytosolic proteins (not sorted to organelle)
• Signal sequence is necessary and sufficient for protein sorting
• Nuclear pores


  Bypass both membranes in nuclear envelope and facilitate bidirectional traffic between cytosol and nucleus
• What the nucleus imports

  • Histones, DNA polymerases, RNA polymerases, transcription factors, and RNA processing proteins

  
• What the nucleus exports

  • Ribosomes, mRNAs, rRNAs, tRNAs and miRNAs
• Nuclear envelope


  • Encloses linear chromosomes and has two membranes (lipid bilayers)
  • Inner nuclear membrane interacts with nuclear lamina
  • Outer nuclear membrane is continuous with ER membrane

  
• Nuclear pores


  • Huge; composed of 1000 proteins (nuclear pore complex)
  • Ribosomal subunits pass through
  • Filled with nucleoporin proteins that create gel in pore
• Small molecules


  Can enter nucleus by diffusion; most proteins and RNA are too large to diffuse passively

  
• Nuclear localization signal (NLS)


  Large proteins must have this correct signal sequence to enter nucleus

  
• Nuclear import


  1. Nuclear import receptor binds to nuclear localization signal on cargo protein in cytoplasm

  2. Nuclear import receptors disrupt interactions between nucleoporin proteins; can pass through using diffusion

  
• Ran


  GTP-binding protein that binds GTP and hydrolyzes it to GDP; Small (<25kDa), so it diffuses through nuclear pore
• Ran GAP


  Enzyme located in cytosol and induces Ran to hydrolyze GTP to GDP (interacts with cytoskeleton)
• Ran GEF


  Enzyme located in nucleus and promotes exchange of GDP to GTP on Ran (interacts with chromatin)

  
• Nuclear import receptors


  1. Use Ran-GTP, Ran GAP, and Ran GEF to determine when to bind and release nuclear protein

  2. Have two binding sites: one for a nuclear protein and one for Ran-GTP; cannot bind to nuclear protein and Ran-GTP at the same time

  
• Critical Steps for Nuclear Import

  1. Receptor binds to sequence signal on protein

  2. Receptor transports protein into nucleus

  3. Receptor binds Ran-GTP; releases protein

  4. Receptor leaves nucleus, enters cytosol

  5. Ran-GTP converted to Ran-GDP; Ran-GDP dissociates from import receptor

  
• Nuclear pores act as selective gates that actively transport specific proteins and molecules
• Ran-GTP, Ran GAP, Ran GEF


  Used by import receptors to determine when to bind and release nuclear protein