Intro to Pharmacology and Toxicology
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The leucine zipper (bZIP) has a dimerization domain as well as a DNA binding domain. It is an amphipathic helix, one face containing hydrophilic residues, the other containing hydrophobic leucines. Dimer formation is due to the hydrophobic effect, driving the leucines together. Each subunit of a bZIP dimer contains basic a DNA binding domain at the C-terminus, the leucine zipper in the middle, and a regulatory domain after the zipper, which interacts with the promoter to stimulate or repress transcription. The C/EBP, AP1 and yeast GCN4 transcription factors are members of the bZIP family.
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Activation Protein 1 (AP-1) transcription factors are heterodimers of the proteins Jun and Fos, both of which have bZIP regions. The dimer binds to the specific cis-element sequence TGA(C/G)TCA to activate transcription of early response genes. Heterodimers of Jun and Fos have the highest activity, although homodimers of Jun have some activity. Homodimers of Fos are inactive.
Jun is activated when phosphorylated by MAP kinase, usually as a result of a growth hormone signaling cascade:
GF 
GFR Ras
Raf MEK
MAPK Jun ELK1Fos |
MAP kinase also phosphorylates ELK1, which in turn stimulates Fos synthesis. Fos and active Jun form heterodimers. Once a homo- or heterodimer is formed, it is kept inactive by phosphorylation by GSR-3 or CK-II, or activated by a phosphatase. The AP-1 heterodimer activates transcription of Jun, therefore acting in a positive autoregulation mechanism.
Another example of bZIP is the cAMP responsive element binding protein (CREB) family, which includes CREB and CREM. They share highly related C-terminus leucine zipper dimerization and basic DNA binding domains. The cAMP responsive element (CRE) to which they bind has a specific sequence: TGACGTCA. All members of this family interact with DNA as homodimers activated by phosphorylation at Ser-133 by PKA (by for example growth factors), and are deactivated by Ser/Thr phosphatases:
GF
GFR cAMP
PKA CREB CRE/CBP
Phosphorylation of CREB at Ser 133 facilitates interaction with CREB binding protein (CBP), which in turn interacts with general transcription factors.
The
helix-loop-helix (bHLH) structure is a dual function domain, and is a
set of two amphipathic helices. It binds to DNA through a region of basic amino
acids (typically 10-13 residues) that precedes the first alpha-helix. The helices
contain hydrophobic amino acids at every third or forth position, so that it
present a hydrophobic surface to the environment. This enables the protein to
pair by hydrophobic interactions with the same protein or with a related protein
that displays such a surface.
There is a ubiquitous bHLH protein synthesized in most cells that can form a dimer with either of two potential partners. One of the possible partners is a positive regulator, the other is a negative regulator. When the positive regulator dimerizes with the ubiquitous protein, it forms an active complex that stimulates transcription from genes it recognizes. When the negative regulator dimerizes with the ubiquitous bHLH, it forms an inhibitory complex that represses transcription of the same genes.
The Myc family of transcription factor are examples of the bHLH and bZIP motifs. Myc itself was discovered as a protein of the avian myclocytoma virus. Other members of the family are Max (Myc activation substance X) and Mad (Max dimerize). This family is a very powerful set of transcription activators and repressors, active all through early mammalian development. They interact with a very specific DNA sequence: CACGTG. All sorts of Homo and heterodimers are possible with this group. Two of the combinations, Max/Max and Mad/Max, function as repressors of transcription while Myc/Max is a strong activator.
When Myc is present, the equilibrium is shifted to Myc/Max dimers (activation), but when Mad is present the equilibrium is shifted to Mad/Max dimers (repression). In this way, the formation of heterodimers control the relative rate of transcription for genes controlled by this family.
The nuclear factor kappa B (NFkB) is a ubiquitously expressed DNA binding protein dimmer (hetero- or homodimer) Its N-terminus binds DNA and the C-terminus is responsible for transcription activation. The mechanism of dimerization and DNA binding are unknown, as NFkB does not belong to either the zinc finger, leucine zipper of bHLH transcription factor families.
NFkB always binds to the consensus sequence GGPuNNPyPyCC, and is constitutively expressed and held inactive in the cytosol by the inhibitory protein IkB, which has a nuclear localization signal mask. Upon stimulation by an intracellular signal, probably a phosphorylation of IkB, the inhibitor dissociates allowing translocation to the nucleus. The stimulatory signal seems to be triggered by inflammatory cytokine receptors.
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