| Regulation of the ETS transcription factor ER 81 by the 90 kDa ribosomal S 6 kinase 1 and protein kinase A . ^^^ Here we describe that the 90 kDa ribosomal S 6 kinase 1 ( RSK 1 ) , a protein kinase downstream of the extracellular signal regulated kinase ( ERK ) subclass of MAPKs , binds to ER 81 , phosphorylates it , and enhances ER 81 dependent transcription . ^^^ Two in vivo RSK 1 phosphorylation sites within ER 81 , Ser ( 191 ) and Ser ( 216 ) , were identified , whose mutation to alanine reduces ER 81 activity upon ERK MAPK stimulation . ^^^ Furthermore , RSK 1 activates the ER 81 cofactor CREB binding protein and may thereby augment ER 81 dependent transcription . ^^^ Similar to RSK 1 , the cAMP dependent protein kinase A ( PKA ) phosphorylates ER 81 on Ser ( 191 ) / Ser ( 216 ) . ^^^ |