One of the most direct pathways of polypeptide-stimuled gene activity
is the JAK-STAT pathway. STATs are so named because they serve both
as signal transducers in the cytoplasm and activators of transcription
in the nucleus. The goal of our research is to define the structural
and biochemical bases of STAT-mediated transcriptional synergism and
the negative regulation of the JAK-STAT pathway.
Different transcriptional activators and DNA-binding proteins have been found
to form macromolecular complexes on clustered DNA-recognition elements in
enhancer and promoter regions. The resulting composite structures are called
enhanceosomes, and they play an important role in synergistic transcriptional
activation. However, the molecular details of their protein-protein and protein-DNA
interactions remain to be elucidated. Recently, several STAT-involved enhanceosomes
have been identified, and they are the targets of our studies.
Inappropriate activation of the STAT proteins has been found in human pathogenesis
and malignancy. An understanding of the way in which the activated STATs
and their upstream proteins are downregulated will aid in the design of anticancer
drugs that target these molecules. Two classes of protein inhibitors have
been found to negatively regulate the JAK-STAT pathway. SOCS (suppressors
of cytokine signaling) and related proteins are SH2-containing molecules
that bind to phosphotyrosine residues on receptors or JAK kinases, preventing
downstream signaling. A second class of inhibitors, called PIAS (protein
inhibitors of the activated STATs), are STAT-specific inhibitors. The PIAS
proteins appear to suppress the DNA-binding activity of STATs, although the
exact mechanism is still unclear. Structures of these protein inhibitors
in complex with appropriate ligands should allow us to elucidate how this
specific inhibition is achieved.
Homeostasis of animals is regulated not only by cell growth and differentiation
but also by programmed cell death (apoptosis). DNA fragmentation factor (DFF)
has recently been found to play an essential role in regulating DNA fragmentation
and chromatin condensation during apoptosis. DFF is a heterodimeric protein
complex: DFF40 possesses a nuclease activity, whereas DFF45 functions as
a molecular chaperone and a specific inhibitor of DFF40. In the event of
an apoptotic signal, DFF45 is cleaved and dissociates from DFF40, allowing
DFF40 to form an oligomeric complex to cleave DNA. Many of the biochemical
details of the DFF pathway remain to be ascertained. Efforts will be made
to crystallize and determine the structures of the pathway’s proteins
and protein complexes.
Representative Publications
- O’Shea JJ, Gadina
M, Chen X (2005) Structure
of a Janus kinase – Molecular insights
and prospects for optimizing a new class of
immunosuppressants. Blood (in
press)
- Mao X, Ren Z, Parker
GN, Sondermann H, Pastorello MA, Wang
W, McMurray JS, Demeler B, Darnell JE
Jr, Chen X (2005) Structural
bases of unphosphorylated STAT1 association
and receptor binding. Molecular
Cell 17,
761–771
- Zhong M, Henriksen
MA, Takeuchi K, Schaefer O, Liu B,
ten Hoeve J, Ren Z, Mao X, Chen
X, Shuai K, Darnell JE
Jr (2005) Implications of an antiparallel
dimeric structure of nonphosphorylated STAT1
for the activation-inactivation cycle. Proceedings
of the National Academy of Sciences, U.S.A. 102,
3966–3971
- O’Shea
JJ, Kanno Y, Chen
X,
Levy DE (2005) Stat acetylation – a
key facet of cytokine signaling? Science 307,
217–218
- Bromberg J, Chen X (2001) STAT proteins:
Signal transducers and activators of transcription. Methods
Enzymol 333, 138–151
- Chen X, Vinkemeier U, Zhao Y, Jeruzalmi
D, Darnell JE Jr, Kuriyan J (1998) Crystal structure
of a tyrosine phosphorylated STAT-1 dimer on
DNA. Cell 93, 827–839
- Chen X, Koshland DE Jr (1997) Probing
the structure of the cytoplasmic domain of the
aspartate receptor by targeted disulfide cross-linking. Biochemistry 36,
11858–11864
- Sano T, Pandori MW, Chen X, Smith CL,
Cantor CR (1995) Recombinant core streptavidins.
A minimum-sized core streptavidin has enhanced
structural stability and higher accessibility
to biotinylated macromolecules. J Biol Chem 270,
28204–28209
- Ito M, Guerriero V Jr, Chen XM, Hartshorne
DJ (1991) Definition of the inhibitory domain
of smooth muscle myosin light chain kinase by
site-directed mutagenesis. Biochemistry 30,
3498–3503
Mailing Address:
Department of Biochemistry and Molecular Biology, Unit 1000
U.T. M .D. Anderson Cancer Center
1515 Holcombe Boulevard
Houston, TX 77030
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