G proteins

在结构上不同于前述的G蛋白，分子量较小，在20-30kDa之间，不是以α、β、γ三聚体方式存在，而是单体分子，因此被称为小G蛋白（small G proteins）。如ras表达产物为一种小G蛋白。小

G proteins

• Superfamilies
  • Trimeric G protein
    • adrenergic signaling
      • Gs
      • Gi
    • vision
      • transducin
  • Small G proteins
    • insulin signaling(Ras)
    • Vesicle trafficking
      • ARF
      • Rab
    • Transport into and out of the nucleus
      • Ran
    • Timing of the cell cycle
      • Rho
    • Protein synthesis
      • initiation factor
        • IF2
      • Elongation factors
        • EF-Tu
        • EF-G
• differences
  • 200 genes
  • differ in
    • size
    • subunit structure
    • intracellular location
    • function
• common
  • timers
    • inactivate themselfs after short period of activation
  • Same core structures
    • active conformation when GTP is bound
    • inactive conformation when GDP is bound
  • many are covalently bond lipids
    • give them an affinity for membranes and dictate their locations in the cell
• Ras as a standard
  • Ras is mutated in 25% of all cancers
    • The mutition site is critical residues around the GTP binding site or in the P loop
      • eliminate the GTPase activity
  • The Swiches
    • switch I
      • Thr 35
    • switch II
      • Gly 60
    • P loop
      • interactes with the γ phosphate of GTP
      • form hydrogen bonds by the phosphate's oxygen with switches I/II
      • So the switch I and II is hided after the γ phosphate is hydrolized
    • Specificity
      • Ala 146 hydrogen-bonds to the guanine oxygen, allowing GTP, but not ATP, to bind
    • Interact with the down stream signals
      • Buried when GDP is bonded
  • Intrinsic GTPase
    • cleaves the γ phosphate
    • Ras
      • GTPase activator proteins(GAPs)
        • Increase the intrinsic GTPase activity for 10^5 folds
        • The tumor suppressor gene NF1 encodes a GAP that enhances the GTPase activity of normal Ras.
          • so the mutation of it will only live Ras with its intrisic GTPase activity and cause cancer
    • heterotrimeric G proteins
      • regulators of G protein signaling (RGSs)
      • Disease causeing mutations
        • Activating mutations in Gs
          • permanently active Ga
            • continueous increase of [AMP]
              • found in about 40% of putuitary tumors(adenomas)
        • Inactivating mutations in Ga
          • unresponsive to hormones
            • such as thyroid hormone
        • Mutations in Transducin α subunit (Tα)
          • involved in visual signaling
            • night blindness
          • due to defective interaction between the activated Tα subunit and the phosphodiesterase of the rod outer segment
        • A sequence variation in the gene encoding the β subunit of a heterotrimeric G protein is commonly found in individuals with hypertension (high blood pressure)
          • this variant gene is suspected of involvement in obesity and athrosclerosis
    • response for the timming
      • They contribute a critical Arg residue that reaches into the G-protein GTPase active site and assists in catalyst
  • Intrinsic replacing bond GDP with GTP
    • Slow
    • Swiching the protein on
    • Catalyzed by guanosine nucleotide exchange factor(GEFs)
      • locates in the upstream signals
• Toxins agains G proteins
  • Cholera toxin
    • secreted by Vibrio cholerae in the intestine of an infected person
    • Heterodimeric protein
    • 1. Entry the cell
      • subunit B recognizes and binds to specific gangliosides on the surface of intestinal epithelial cells and provides a route for subunit A to enter these cells.
    • 2. After entry
      • After intry, subunit A is brocken into two pieces: the A1 fragment and A2 fragment.
        • A1 associates with ADP-ribosylation factor ARF6 (a small G protein) through residues in its swich I and swich II regions.
          • ARF6 is only accessible in its active(GTP bound form)
          • The association with ARF6 activates A1, which catalyzes the transfer of ADP-ribose from NAD+ to critical Arg residue in the P loop of the α subunit of Gs.
          • ADP-ribosylation blocks the GTPase activity of Gs and thereby renders Gs permanently active.
            • This results in
              • continous activation of the adenylyl cyclase of intestinal epicelial cells
              • chronically high [cAMP]
              • chronically active PKA.
                • PKA phosphprylates the CFTR Cl- channel and a Na+-H+ echanger in the intestinal epithelial cells.
                • It results in efflux of NaCl
                  • It triggers massive water loss through the intestine
                  • To ensuring osmotic imbalance
                    • severe dehydration
                    • severe electrolytes lost
  • Pertussis toxin
    • Produced by Bordetella pertussis
    • catalyzes ADP-ribosylation of the α subunit of Gi
      • Prevents GDP-GTP exchange
      • Blocks the inhibition of adenylyl cyclase by Gi
    • The bacteria infects the respiratory tract
      • it destroys the ciliated epithelial cells that normally sweep away mucus.
        • Without this ciliary action, vigorous coughing is needed to clear the tract.
          • Gasping cough and spreads the bacteria to others