Molecular dynamics simulations of the conformational changes in signal transducers and activators of transcription, Stat1 and Stat3

All signal transducers and activators of transcription (STAT) factors are a family of cytoplasmic transcription factors that mediate the signal response to cytokines, growth factors, and hormonal factors. The phosphorylation and subsequent activation of Stat3, a member of the STAT family, has been found to be elevated in a large number of diverse human cancers. Understanding of the dynamics of the Stat3 dimer interface is pertinent to designing small molecule inhibitors to activated Stat3 dimer. To this end, we have performed molecular dynamics simulations in explicit water of the activated Stat3 homodimer, and also its closely related member of the STAT family, activated Stat1 homodimer. We observed a large-scale domain motion in the Stat3 dimer while the structure of the monomer remains intact. The driving force for this conformational change is enhanced binding of the Stat3 dimer to the DNA, thereby regulating gene expression. Our model shows that the carboxy terminus of one monomer wraps around the core of the SH2 domain of the other monomer, and this region that makes up the dimer interface remains intact during the dynamics. Water diffuses into a cavity under this dimer interface, thus expanding a pre-existing cavity that gets gated and closed by the loops in the SH2 domain. This cavity could serve as a potential binding pocket for inhibitor design.