Reiterated activation of MAPK is a key step in a wide array of developmental processes, where often MAPK activates transcription of down stream genes. However, the potential activity of MAPK to influence processes involved in RNA metabolism has yet to be elucidated. Here we show that activated MAPK affects RNA metabolism by specific phosphorylation of the RNA binding protein Held Out Wings (HOW).
Drosophila HOW is a conserved RNA-binding protein (RBP) belonging to the STAR family, whose closest mammalian ortholog Quaking, has been implicated in embryonic development and nervous system myelination. The HOW RBP modulates a variety of developmental processes by controlling mRNA levels and the splicing profile of multiple key regulatory factors. We show that HOW undergoes phosphorylation by MAPK/ERK, and that this modification facilitates its dimerization. Employing an antibody that specifically recognizes phosphorylated HOW, we show that HOW is phosphorylated in-vivo in embryonic muscle cells and heart cardioblasts. We further show that HOW regulates sallimus mRNA levels in a phosphorylation-dependent manner, underscoring the physiological relevance of this modification. Taken together, our results provide a detailed mechanism of HOW activation; MAPK/ERK-dependent phosphorylation of HOW promotes the formation of HOW dimers, which are presumably more active in controlling mRNA levels of key muscle-specific factors, and consequently, differentiation of this tissue.