Forkhead in rhabdomyosarcoma (FKHR) is a transcription factor that has been implicated in the control of gene expression by insulin, as well as the regulation of apoptosis by survival factors. These signals trigger the protein kinase B (PKB)-catalysed phosphorylation of FKHR at three residues (Thr24, Ser256 and Ser319) by a phosphoinositide 3-kinase-dependent pathway that results in the nuclear exit and inactivation of this transcription factor. Here, we have identified a conserved residue (Ser329) as a novel in vivo phosphorylation site on FKHR. Ser329 phosphorylation also decreases the ability of FKHR to stimulate gene transactivation and reduces the proportion of FKHR present in the nucleus. However, unlike the residues targetted by PKB, Ser329 is phosphorylated in unstimulated HEK-293cells, and phosphorylation is not increased by stimulation with insulin-like growth factor-1 or by transfection with 3-phosphoinositide-dependent protein kinase-1. We have also purified a protein kinase to near homogeneity from rabbit skeletal muscle that phosphorylates FKHR at Ser329 specifically and identified it as DYRK1A (dual-specificity tyrosine-phosphorylated and regulated kinase 1A). We find that FKHR and DYRK1A co-localize in discrete regions of the nucleus and can be co-immunoprecipitated from cell extracts. These experiments suggest that DYRK1A may phosphorylate FKHR at Ser329 in vivo.
- insulin action
- gene transcription
- protein kinase B
- The Biochemical Society, London © 2001