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with the cytoplasmic portion of the receptor as described above. Thus, the orientation of the C-terminal
portion of the IFN-γ molecules in the receptor complex should be such that they are situated near to the
cell membrane. When one examines the structure of the receptor-ligand complex, it is easy to see that
this is indeed the case. Studies are currently under way to determine the crystal structure of the complex
between the cytoplasmic domain of the IFN-γ receptor and the IFN-γ molecule, in particular the Cterminus.
With regards to the definition of sites of interaction between receptors and ligands, the
synthetic peptide approach has repeatedly proven to be an accurate indicator of structurally important
regions of the IFN-γ/IFN-γ R system.
D. Signal Transduction by IFN-γ
Within the past several years some of the immediate-early signal transduction events initiated in
response to IFN-γ stimulation have been elucidated. Treatment of cells with IFN-γ leads to the rapid
activation of two protein tyrosine kinases, JAK1 and JAK2 [47]. The JAK kinases are a newly emerging
family of protein kinases important in signaling via cytokines and growth factors. These proteins are
unrelated to the src family of tyrosine kinases and are characterized as being larger, having two putative
phosphotransferase domains and containing no characteristic SH2 or SH3 domains [48–51]. Members of
the Janus kinase family are found associated with the cytoplasmic domains of cytokine and growth
factor receptors at or near to the membrane proximal region [51].
In the resting cell, JAK1 and JAK2 are found associated with the α and β/AF-1 chains of the IFN-γ
receptor, respectively [52]. These kinases as well as the ligand-binding chain of the IFN-γ receptor are
tyrosine phosphorylated in response to IFN-γ treatment [47,53,54]. This leads in turn to the tyrosine
phosphorylation of a latent cytoplasmic transcription factor, known variously as p91, Stat 91, or Stat 1 α
on tyrosine residue 701 [55]. It is interesting to note that the IFN-α signal-transduction pathway partially
overlaps with that of IFN-γ. Stimulation of cells by IFN-α leads to the activation of JAK1 and another
Janus family kinase, Tyk2 [56,57]. In turn, this cascade leads to phosphorylation of two latent
cytoplasmic transcription factors, p84 (Stat 1β) and p113 (Stat 2β) in addition to the p91 (Stat 1α)
activated by IFN-γ.
The identification of tyrosine kinases that directly associate with the subunits of the IFN-γ receptor lead
to the question of how the binding of IFN-γ might affect these proteins. Recently, the synthetic peptide
method was used to identify two regions of the murine IFN-γ receptor's α chain as being important for
interaction with the kinase JAK2 [58]. One of these regions lies in the distal portion of the cytoplasmic
tail (residues 404–432), while the other (residues 283–309) is nearer to the membrane proximal region to
which the C-terminal part of IFN-γ binds (residues 253–287). The fact that there are adjacent binding
sites for JAK2 and IFN-γ implied a potential for interaction between the IFN-γ
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