The Essential Role of PP1β inDrosophilaIs to Regulate Nonmuscle Myosin

Abstract

Reversible phosphorylation of myosin regulatory light chain (MRLC) is a key regulatory mechanism controlling myosin activity and thus regulating the actin/myosin cytoskeleton. We show that Drosophila PP1beta, a specific isoform of serine/threonine protein phosphatase 1 (PP1), regulates nonmuscle myosin and that this is the essential role of PP1beta. Loss of PP1beta leads to increased levels of phosphorylated nonmuscle MRLC (Sqh) and actin disorganisation; these phenotypes can be suppressed by reducing the amount of active myosin. Drosophila has two nonmuscle myosin targeting subunits, one of which (MYPT-75D) resembles MYPT3, binds specifically to PP1beta, and activates PP1beta's Sqh phosphatase activity. Expression of a mutant form of MYPT-75D that is unable to bind PP1 results in elevation of Sqh phosphorylation in vivo and leads to phenotypes that can also be suppressed by reducing the amount of active myosin. The similarity between fly and human PP1beta and MYPT genes suggests this role may be conserved.

The Nonmuscle Myosin Phosphatase PP1β (flapwing) Negatively Regulates Jun N-Terminal Kinase in Wing Imaginal Discs of Drosophila

“…bsk 1 also suppressed the strong and semilethal allele flw 7 (not shown). Regulation of nonmuscle myosin activity is the only essential, nonredundant function of flw in Drosophila (Vereshchagina 2004). The fact that bsk could suppress flw 6 , albeit more weakly than, for example, the flw Flw is a negative regulator of puc expression in wing imaginal discs: The allele puc A251 is due to a P{lArB} insertion in the second intron of puc ( Figure 1E) and has also been widely used as a puc enhancer trap, as the expression of the lacZ gene on the P element is under the control of a genomic puc enhancer (Dobens 2001).…”

“…Flw, but not PP1a, binds specifically to the myosin phosphatase-targeting subunit MYPT-75D, the homologue of mammalian MYPT3 (Vereshchagina 2004). Recently, phosphorylation of human MYPT3 was shown to activate MYPT3-associated PP1d toward cytoplasmic myosin regulatory light chain (Yong 2006).…”

“…Reducing the amount of active Zip suppresses the semilethality of flw 6 , indicating that regulation of nonmuscle myosin activity is the only essential function of flw. Conversely, expressing a constitutively active form of Sqh enhances the weak viable allele flw 1 toward complete lethality (Vereshchagina 2004).…”

“…Another myosin phosphatase-targeting subunit in Drosophila is the myosin-binding subunit (Mbs), which is the homologue of mammalian MYPT1/2; it binds both Flw and Pp1-87B in vitro (Vereshchagina 2004). The level of P-Sqh is elevated in Mbs mutant clones (Lee and Treisman 2004).…”

“…This suggests that Mbs targets PP1c to its substrate Sqh, and therefore flw might not be the only Sqh phosphatase. However, it is likely that the two presumptive nonmuscle myosin phosphatases Mbs/ PP1c and MYPT-75D/Flw are localized differentially in cells because MYPT-75D has a prenylation motif while Mbs does not (Vereshchagina 2004). This, together with the specificity of MYPT-75D for Flw, may account for the evident lack of redundancy between Flw and other potential Sqh phosphatases.…”