The nonmuscle myosin phosphatase PP1[beta] (flapwing) negatively regulates Jun N-terminal kinase in wing imaginal discs of Drosophila

Drosophilaflapwing (flw) codes for serine/threonine protein phosphatase type 1[beta] (PP1[beta]). Regulation of nonmuscle myosin activity is the single essential flw function that is nonredundant with the three closely related PP1[alpha] genes. Flw is thought to dephosphorylate the nonmuscle myosin regulatory light chain, Spaghetti Squash (Sqh); this inactivates the nonmuscle myosin heavy chain, Zipper (Zip). Thus, strong flw mutants lead to hyperphosphorylation of Sqh and hyperactivation of nonmuscle myosin activity. Here, we show genetically that a Jun N-terminal kinase (JNK) mutant suppresses the semilethality of a strong flw allele. Alleles of the JNK phosphatase puckered (puc) genetically enhance the weak allele [flw.sup.1], leading to severe wing defects. Introducing a mutant of the nonmuscle myosin-binding subunit (Mbs) further enhances this genetic interaction to lethality. We show that puc expression is upregulated in wing imaginal discs mutant for [flw.sup.1] and [puc.sup.A251] and that this upregulation is modified by JNK and Zip. The level of phosphorylated (active) JNK is elevated in [flw.sup.1] enhanced by puc. Together, we show that disruption of nonmuscle myosin activates JNK and puc expression in wing imaginal discs.