239 : Dynamic nuclear trafficking of STAT5 regulated by an unconventional nuclear localization signal

Abstract

As a regulated transcription factor, precise cellular localization of STAT5 is essential for physiological processes that include hematopoiesis, liver metabolism, and mammary gland development. Conventional nuclear localization signals consist of short stretches of basic amino acids. We provide evidence that STAT5 nuclear import is dependent on an unconventional nuclear localization signal that functions within the conformation of an extensive coiled-coil domain. Both in vitro binding and in vivo functional assays reveal that STAT5 nuclear import is mediated by the importin- α 3/ β 1 system independent of STAT5 activation by tyrosine phosphorylation. The integrity of the coiled-coil domain is essential for STAT5 transcriptional induction of the β -casein gene following prolactin stimulation, as well as its ability to synergize with the glucocorticoid receptor. The glucocorticoid receptor accumulates in the nucleus in response to prolactin, and this nuclear import is dependent on STAT5 nuclear import. STAT5 continually shuttles in and out of the nucleus, and live cell imaging demonstrates STAT5 nuclear export is mediated by both Crm1-dependent and Crm1-independent pathways. A Crm1-dependent nuclear export signal was identified within the STAT5 amino terminus. These findings provide insight to fundamental mechanisms that regulate STAT5 nuclear trafficking and cooperation with the glucocorticoid receptor, and provide a basis for clinical intervention of STAT5 function in disease.