Protein phosphatase 1βis required for the maintenance of muscle attachments

Type 1 serine/threonine protein phosphatases (PP1) are important regulators of many cellular and developmental processes, including glycogen metabolism, muscle contraction, and the cell cycle [1-5]. Drosophila and humans both have multiple genes encoding PP1 isoforms [3,6,7]; each has one β and several a isoform genes (α1, α2, α3 in flies, α and γ in humans; mammalian PP1β is also known as PP1δ). The α/β subtype differences are highly conserved between flies and mammals [6]. Though all these proteins are <85% identical to each other and have indistinguishable activities in vitro, we show here that the Drosophila β isoform has a distinct biological role. We show that PP1β9C corresponds to flapwing (flw), previously identified mutants of which are viable but flightless because of defects in indirect flight muscles (IFMs) [8]. We have isolated a new, semi-lethal flw allele that shows a range of defects, especially in muscles, which break away from their attachment sites and degenerate