||We previously demonstrated that the Epstein-Barr virus-encoded latent membrane protein 1 (LMP1) potently activates the cellular c-Jun N-terminal kinase (JNK) pathway by sequentially engaging an unknown adaptor, TRAF6, TAB1/TAK1, and JNKKs. We now show that BS69, a MYND domain-containing cellular protein, is the missing adaptor that bridges LMP1 and TRAF6, as the MYND domain and a separate region of BS69 bind to the carboxyl termini of LMP1 and TRAF6, respectively. While LMP1 promotes the interaction between BS69 and TRAF6, the complex formation between LMP1 and TRAF6 is BS69 dependent. A fraction of LMP1 and BS69 is constitutively colocalized in the membrane lipid rafts. Importantly, knockdown of BS69 by small interfering RNAs specifically inhibits JNK activation by LMP1 but not tumor necrosis factor alpha. Although overexpression of either BS69 or a mutant LMP1 without the cytoplasmic carboxyl tail is not sufficient to activate JNK, interestingly, when BS69 is covalently linked to the mutant LMP1, the chimeric protein restores the ability to activate JNK. This indicates that the recruitment and aggregation of BS69 is a prerequisite for JNK activation by LMP1. Unexpectedly, we show that BS69 is also involved in the p53-p21Cip1-mediated senescence pathway. Knockdown of BS69 by RNA interference in primary human fibroblasts results in elevated p21Cip1 levels and appearance of a number of senescent markers including enhanced senescence-associated β-galactosidase activity and formation of senescence-associated heterochromatin foci. Importantly, knockdown of either p53 or p21Cip1, but not p16INK4a or Rb, allows cells to bypass premature senescence induced by BS69 knockdown. Furthermore, we demonstrate that BS69 forms complexes with both p53 and p400 and that BS69 associates with the p21Cip1 promoter through p53. Taken together, our data suggest that BS69 plays diverse roles in cells: it can serve not only as an adaptor in the LMP1-mediated JNK pathway, but also as a regulator in the cellular senescence pathway.