The Nonmuscle Myosin Phosphatase Pp1 Beta (Flapwing) Negatively Regulates Jun N-Terminal Kinase In Wing Imaginal Discs Of Drosophila

Drosophila flapwing(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(I), 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(I) and puc(125I) and that this upregulation is modified by JNK and Zip. The level of phosphorylated (active) JNK is elevated in flw(I) enhanced by puc. Together, we show that disruption of nonmuscle myosin activates JNK and Puc expression in wing imaginal discs.