Post-translational modifications of BRAF and MITF

Malignant melanoma is the deadliest and most aggressive form of skin cancer. Despite the development of targeted molecular therapies which specifically target oncogenic pathways in melanoma, melanoma remains highly refractory to treatment and prone to relapse. In order to develop more effective therapies, there is a need to investigate additional ways of manipulating aberrant molecular pathways in melanoma. To this end, we have identified novel sites of post-translational modifications in two oncogenic proteins that are known to play pivotal roles in driving melanoma tumorigenesis. We showed that BRAF, the most commonly mutated oncoprotein in melanoma, can be acetylated at K473 and K475 by the p300/CBP acetyltransferases. Importantly, acetylation of BRAF reduced its activity regardless of its mutational status at the commonly mutated V600 residue. We also identified a novel phosphorylation site targeted by GSK3 in microphthalmia-associated transcription factor (MITF), the melanocyte master regulator. GSK3 phosphorylation of S69, together with ERK-mediated phosphorylation of the nearby S73 residue, was found to promote MITF nuclear export via a previously undescribed nuclear export signal comprising of the S69, S73, M75, L78 and L80 residues. Importantly, phosphorylation-induced nuclear export was associated with reduced MITF activity, which may have important functional implications for melanocyte development and melanoma oncogenesis. In addition, we showed that the cyclin-dependent kinases CDK1 and CDK2 can also phosphorylate MITF at S73.