Phosphorylation of MafA Is Essential for Its Transcriptional and Biological Properties

Abstract

We previously described the identification of quail MafA, a novel transcription factor of the Maf bZIP (basic region leucine zipper) family, expressed in the differentiating neuroretina (NR). In the present study, we provide the first evidence that MafA is phosphorylated and that its biological properties strongly rely upon phosphorylation of serines 14 and 65, two residues located in the transcriptional activating domain within a consensus for phosphorylation by mitogen-activated protein kinases and which are conserved among Maf proteins. These residues are phosphorylated by ERK2 but not by p38, JNK, and ERK5 in vitro. However, the contribution of the MEK/ERK pathway to MafA phosphorylation in vivo appears to be moderate, implicating another kinase. The integrity of serine 14 and serine 65 residues is required for transcriptional activity, since their mutation into alanine severely impairs MafA capacity to activate transcription. Furthermore, we show that the MafA S14A/S65A mutant displays reduced capacity to induce expression of QR1, an NR-specific target of Maf proteins. Likewise, the integrity of serines 14 and 65 is essential for the MafA ability to stimulate expression of crystallin genes in NR cells and to induce NR-to-lens transdifferentiation. Thus, the MafA capacity to induce differentiation programs is dependent on its phosphorylation.

CD13/APN transcription is regulated by the proto-oncogene c-Maf via an atypical response element

“…5B). The observed mobilities are comparable to those described for the analogous L-Maf mutants where proteins harboring the S65A mutation showed faster mobility than either wild type or those containing the S14A alteration (Benkhelifa et al, 2001) and are independent of the phosphorylation status of the protein.…”

“…We find that alteration of the Ser 15 and Ser 70 residues increases c-Maf activity on the CD13 promoter in the HepG2 liver cell line and thus are inhibitory. However others have found that the S14A/S65A double mutant of L-Maf significantly reduced L-Maf activity (Ochi et al, 2003,Benkhelifa et al, 2001. The mechanisms regulating activation of Maf family proteins have been investigated (Gardner and Montminy, 2005) and include a model invoking bFGF/MEK/ERK-induced phosphorylation that triggers proteosome-dependent degradation (Ochi et al, 2003) to explain L-Maf's decreased stability in neural retinal cells.…”

Inhibition of MafA transcriptional activity and human insulin gene transcription by interleukin-1β and mitogen-activated protein kinase kinase kinase in pancreatic islet beta cells

“…However, in the present study overproduction of MEKK1 did not decrease the amount of MafA produced in JEG cells (data not shown), arguing against a MEKK1-induced degradation of MafA. In addition to protein synthesis or degradation, the transcriptional activity of MafA is regulated by phosphorylation [46]. The present study suggests that IL-1β through activation of MEKK1 phosphorylates MafA or an associated protein, thereby resulting in a decrease of human insulin gene transcription.…”

“…Thus, MafA transcriptional activity is regulated by MEKK1 and by IL-1β. MafA belongs to the class of basic region/leucine zipper transcription factors and has been implicated in the development and differentiation of the lens [46]. Moreover, identification of MafA as the transcription factor binding to the C1 element within the rat Ins2 and the INS promoter [16,17] fuelled studies on the role of MafA in beta cell function.…”

Mouse MafA, homologue of zebrafish somite Maf 1, contributes to the specific transcriptional activity through the insulin promoter

“…Recently, it has been published that chicken L-Maf, which is the homologue of mouse MafA, is phosphorylated by FGF/ERK signaling pathways and that phosphorylation of L-Maf induces its degradation [52]. In addition, it has been reported that quail MafA is phosphorylated and its phosphorylation is important for maximum transactivation [53]. These data suggest that mouse MafA must also be phosphorylated and that its phosphorylation may regulate the transcriptional activity and/or stability of MafA by extracellular signaling, glucose.…”