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 both differentiated features of melanocytes such as pigmentation as well as proliferation/survival, based on phenotypes of mutant mouse alleles. Mi activity is controlled by at least two signaling pathways. Melanocyte-stimulating hormone (MSH) promotes transcription of the Mi gene through cAMP elevation, resulting in sustained Mi up-regulation over many hours. c-Kit signaling up-regulates Mi function through MAP kinase phosphorylation of Mi, thereby recruiting the p300 transcriptional coactivator. The current study reveals that c-Kit signaling triggers two phosphorylation events on Mi, which up-regulate transactivation potential yet simultaneously target Mi for ubiquitin-dependent proteolysis. The specific activation/degradation signals derive from MAPK/ERK targeting of serine 73, whereas serine 409 serves as a substrate for p90 Rsk-1. An unphosphorylatable double mutant at these two residues is at once profoundly stable and transcriptionally inert. These c-Kit-induced phosphorylations couple transactivation to proteasome-mediated degradation. c-Kit signaling thus triggers short-lived Mi activation and net Mi degradation, in contrast to the profoundly increased Mi expression after MSH signaling, potentially explaining the functional diversity of this transcription factor in regulating proliferation, survival, and differentiation in melanocytes.
Wu
M
M
Division of Pediatric Oncology, Dana-Farber Cancer Institute, Harvard Medical School, Boston, Massachusetts 02115, USA.
Hemesath
T J
TJ
Takemoto
C M
CM
Horstmann
M A
MA
Wells
A G
AG
Price
E R
ER
Fisher
D Z
DZ
Fisher
D E
DE
D000818:Animals; D015153:Blotting, Western; D002478:Cells, Cultured; D006224:Cricetinae; D003546:Cysteine Endopeptidases / Q000378:metabolism; D004268:DNA-Binding Proteins / Q000378:metabolism*; D006801:Humans; D008544:Melanocytes / Q000378:metabolism*; D051379:Mice; D051739:Microphthalmia-Associated Transcription Factor; D020928:Mitogen-Activated Protein Kinases / Q000235:genetics / Q000378:metabolism; D009097:Multienzyme Complexes / Q000378:metabolism; D009154:Mutation; D010766:Phosphorylation; D046988:Proteasome Endopeptidase Complex; D019009:Proto-Oncogene Proteins c-kit / Q000378:metabolism*; D038744:Ribosomal Protein S6 Kinases, 90-kDa; D019089:Stem Cell Factor / Q000378:metabolism; D014157:Transcription Factors; D015533:Transcriptional Activation; D014452:Ubiquitins / Q000378:metabolism
D004268:DNA-Binding Proteins; C497234:MITF protein, human; D051739:Microphthalmia-Associated Transcription Factor; C497235:Mitf protein, mouse; D009097:Multienzyme Complexes; D019089:Stem Cell Factor; D014157:Transcription Factors; D014452:Ubiquitins; D019009:Proto-Oncogene Proteins c-kit; C069329:RPS6KA1 protein, human; D038744:Ribosomal Protein S6 Kinases, 90-kDa; C512964:Rps6ka1 protein, mouse; D020928:Mitogen-Activated Protein Kinases; D003546:Cysteine Endopeptidases; D046988:Proteasome Endopeptidase Complex