MafB protein stability is regulated by the JNK and ubiquitin-proteasome pathways

MafB is a basic leucine zipper transcription factor that plays important roles in development and differentiation processes. During osteoclastogenesis, its expression is downregulated at the transcriptional level via the JNK and p38 MAP kinase pathways. In the present study, we demonstrated that MafB protein stability is regulated by JNK and identified a phosphorylation site, Thr62. The expression of a constitutively active form of JNK (a fusion protein MKK7alpha1-JNK1beta1) promoted the degradation of MafB in COS7 cells, and a T62A substitution significantly reduced the instability of MafB. The introduction of a fourfold (T58A/T62A/S70A/S74A) substitution in an acidic transcription-activating domain almost protected the instability resulting from the activation of JNK. Furthermore, treatment with proteasome inhibitors increased the MafB level, and a high-molecular-weight smear, characteristic of polyubiquitination, was observed in lysates from cells in which MafB, ubiquitin, and MKK7alpha1-JNK1beta1 were co-expressed. These results suggest that phosphorylation of MafB by JNK confers susceptibility to proteasomal degradation.