c-Jun N-terminal kinase (JNK) phosphorylates Ser62 of Bcl-xL to induce degradation by the ubiquitin proteasome pathway inWEHI-231 cells upon BCR crosslinking. To elucidate the mechanism underlying this regulation of Bcl-xL phosphorylation, we prepared a system in which JNK phosphorylated Bcl-xLinHEK293T cells. This phosphorylationwas Ser62-specific, because the phosphorylation of the Ser62Ala (S62A)mutant formof Bcl-xL was not observed with a pSer62-specific antibody. We found that a signal transductionmolecule,alpha4, enhanced the phosphorylation of Bcl-xL by JNK.Alpha4 associated both with JNK and Bcl-xL, and was mainly localized in cytoplasm. Therefore, alpha4 may regulate the activity of JNK toward Bcl-xLin cytoplasm.