c-Kit triggers dual phosphorylations, which couple activation and degradation of the essential melanocyte factor Mi

Microphthalmia (Mi) is a bHLHZip transcription factor that is essential for melanocyte development and postnatal function. It is thought to regulate b oth differentiated features of melanocytes such as pigmentation as well as proliferation/survival, based on phenotypes of mutant mouse alleles. Mi act ivity is controlled by at least two signaling pathways. Melanocyte-stimulat ing hormone (MSH) promotes transcription of the Mi gene through cAMP elevat ion, resulting in sustained Mi up-regulation over many hours, c-Kit signali ng up-regulates Mi function through MAP kinase phosphorylation of Mi, there by recruiting the p300 transcriptional coactivator. The current study revea ls that c-Kit signaling triggers two phosphorylation events on Mi, which up -regulate transactivation potential yet simultaneously target Mi for ubiqui tin-dependent proteolysis. The specific activation/degradation signals deri ve from MAPK/ERK targeting of serine 73, whereas serine 409 serves as a sub strate for p90 Rsk-1. An unphosphorylatable double mutant at these two resi dues is at once profoundly stable and transcriptionally inert. These c-Kit- induced phosphorylations couple transactivation to proteasome-mediated degr adation, c-Kit signaling thus triggers short-lived Mi activation and net Mi degradation, in contrast to the profoundly increased Mi expression after M SH signaling, potentially explaining the functional diversity of this trans cription factor in regulating proliferation, survival, and differentiation in melanocytes.