Essential, overlapping and redundant roles of the Drosophila protein phosphatase 1 alpha and 1 beta genes.

Protein serine/threonine phosphatase type 1 (PP1) has been found in all eukaryotes examined to date and is involved in the regulation of many cellular functions, including glycogen metabolism, muscle contraction, and mitosis. In Drosophila, four genes code for the catalytic subunit of PP1 (PP1c), three of which belong to the PP1 alpha subtype. PP1 beta 9C (flapwing) encodes the fourth PP1c gene and has a specific and nonredundant function as a nonmuscle myosin phosphatase. PP1 alpha 87B is the major form and contributes approximately 80% of the total PP1 activity. We describe the first mutant alleles of PP1 alpha 96A and show that PP1 alpha 96A is not an essential gene, but seems to have a function in the regulation of nonmuscle myosin. We show that overexpression of the PP1 alpha isozymes does not rescue semilethal PP1 beta 9C mutants, whereas overexpression of either PP1 alpha 96A or PP1 beta 9C does rescue a lethal PP1 alpha 87B mutant combination, showing that the lethality is due to a quantitative reduction in the level of PP1c. Overexpression of PP1 beta 9C does not rescue a PP1 alpha 87B, PP1 alpha 96A double mutant, suggesting an essential PP1 alpha-specific function in Drosophila.