| In this study , we identified a novel interaction partner of ACTR , the ETS transcription factor ER 81 that is also heavily implicated in mammary tumor formation . ^^^ ACTR and related p 160 family members ( steroid receptor coactivator 1 and glucocorticoid receptor interacting protein 1 ( GRIP 1 ) ) augment ER 81 mediated transcription . ^^^ Although ACTR and GRIP 1 can acetylate ER 81 , this posttranslational modification of ER 81 is not required for its stimulation by ACTR or GRIP 1 . ^^^ In addition , ACTR collaborates with the p 300 coactivator , a joint interaction partner of ACTR and ER 81 , to stimulate ER 81 function and the ability of p 300 to acetylate ER 81 is indispensable for this collaboration . ^^^ Furthermore , the receptor tyrosine kinase HER2 / Neu , an oncoprotein particularly found overexpressed in breast tumors , cooperates with both ACTR and p 300 to stimulate ER 81 mediated transcription . ^^^ |