title: The role of the N-terminal domain in dimerization and nucleocytoplasmic shuttling of latent STAT3.
abstract: STAT3 is an important transcription factor involved in immunity and cancer. In response to cytokine stimulation, STAT3 becomes phosphorylated on a single tyrosine residue. Tyrosine-phosphorylated STAT3 accumulates in the nucleus, binds to specific DNA response elements and induces gene expression. Unphosphorylated, latent STAT3 shuttles constitutively between cytoplasm and nucleus. We analysed the importance of previously identified putative nuclear localization sequences (NLS) and nuclear export sequences (NES) for nucleocytoplasmic shuttling of latent STAT3 using STAT3-deficient cells reconstituted with fluorescently labelled STAT3 mutants. Mutation of a putative NLS or NES sequence did not impair nucleocytoplasmic shuttling of latent STAT3. We were also interested in the structural requirements for dimerization of unphosphorylated STAT3 and its relevance for nucleocytoplasmic shuttling. By native gel electrophoresis and dual-focus fluorescence correlation spectroscopy (2f-FCS) we identified the N-terminal domain (amino acids 1-125) to be essential for formation of unphosphorylated STAT3 dimers but not for assembly of tyrosine-phosphorylated STAT3 dimers. In resting cells, the monomeric N-terminal deletion mutant (STAT3-ΔNT) shuttles faster between the cytoplasm and nucleus than the wild-type STAT3, indicating that dimer formation is not required for nucleocytoplasmic shuttling of latent STAT3. STAT3-ΔNT becomes phosphorylated and dimerizes in response to interleukin-6 stimulation but, surprisingly, does not accumulate in the nucleus. These results highlight the importance of the N-terminal domain in the formation of unphosphorylated STAT3 dimers and nuclear accumulation of STAT3 upon phosphorylation.
authors.affiliation: Institute of Biochemistry and Molecular Biology, Medical School, RWTH Aachen University, Pauwelsstrasse 30, Aachen 52074, Germany.
authors.forename: Michael
authors.initials: M
authors.lastname: Vogt
authors.forename: Tamas
authors.initials: T
authors.lastname: Domoszlai
authors.forename: Dzina
authors.initials: D
authors.lastname: Kleshchanok
authors.forename: Swen
authors.initials: S
authors.lastname: Lehmann
authors.forename: Anne
authors.initials: A
authors.lastname: Schmitt
authors.forename: Valeria
authors.initials: V
authors.lastname: Poli
authors.forename: Walter
authors.initials: W
authors.lastname: Richtering
authors.forename: Gerhard
authors.initials: G
authors.lastname: Müller-Newen
mesh_terms: D021581:Active Transport, Cell Nucleus; D000818:Animals; D002460:Cell Line; D002467:Cell Nucleus / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D003593:Cytoplasm / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D019281:Dimerization; D051379:Mice; D010766:Phosphorylation; D017434:Protein Structure, Tertiary; D050796:STAT3 Transcription Factor / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*
chemical_list: D050796:STAT3 Transcription Factor