1 The non - muscle myosin phosphatase PP 1 β ( flapwing ) negatively regulates JNK in wing imaginal discs of Drosophila

Drosophila flapwing (flw) codes for serine/threonine protein phosphatase type 1β (PP1β). Regulation of non-muscle myosin activity is the single essential flw function that is nonredundant with the three closely related PP1α genes. Flw is thought to dephosphorylate the nonmuscle myosin regulatory light chain, Spaghetti Squash (Sqh); this inactivates the non-muscle myosin heavy chain, Zipper (Zip). Thus, strong flw mutants lead to hyperphosphorylation of Sqh and hyperactivation of non-muscle myosin activity. Here, we show genetically that a Jun Nterminal kinase (JNK) mutant suppresses the semi-lethality of a strong flw allele. Alleles of the JNK phosphatase puckered (puc) genetically enhance the weak allele flw, leading to severe wing defects. Introducing a mutant of the non-muscle 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 and puc, and that this upregulation is modified by JNK and Zip. The level of phosphorylated (active) JNK is elevated in flw enhanced by puc. Together, we show that disruption of non-muscle myosin activates JNK and puc expression in wing