Ser298 of MITF, a mutation site in Waardenburg syndrome type 2, is a phosphorylation site with functional significance

MITF (microphthalmia-associated transcription factor) is a basic-helix-loop -helix-leucine zipper (bHLHZip) factor which regulates expression of tyrosi nase and other melanocytic genes via a CATGTG promoter sequence, and is inv olved in melanocyte differentiation. Mutations of MITF in mice or humans wi th Waardenburg syndrome type 2 (WS2) often severely disrupt the bHLHZip dom ain, suggesting the importance of this structure. Here, we show that Ser298 , which locates downstream of the bHLHZip and was previously found to be mu tated in individuals with WS2, plays an important role in MITF function. Gl ycogen synthase kinase 3 (GSK3) was found to phosphorylate Ser298 in vitro, thereby enhancing the binding of MITF to the tyrosinase promoter. The same serine was found to be phosphorylated in vivo, and expression of dominant- negative GSK3 beta selectively suppressed the ability of MITF to transactiv ate the tyrosinase promoter. Moreover, mutation of Ser298, as found in a WS 2 family, disabled phosphorylation of MITF by GSK3 beta and impaired MITF f unction. These findings suggest that the Ser298 is important for MITF funct ion and is phosphorylated probably by GSK3 beta.