SDS22 selectively recognizes and traps metal-deficient inactive PP1

Significance PP1, a metal-dependent enzyme responsible for >50% of all dephosphorylation reactions, forms distinct holoenzymes with >200 regulatory proteins, which create specificity for its substrates. Intriguingly, one of its most ancient regulators, SDS22, is not only an essential gene but also is both a PP1 inhibitor and an activator. How such divergent functions are mediated by the same regulator is unknown. Here, we discovered that SDS22 binds a unique, metal-deficient conformation of PP1, which renders PP1 inactive. Furthermore, once the complex forms, it does not permanently dissociate until PP1 binds a M1 metal. Thus, SDS22 is a "PP1 trap," providing a pool of PP1 poised for the rapid formation of new holoenzymes in response to dynamically changing cellular events. The metalloenzyme protein phosphatase 1 (PP1), which is responsible for ≥50% of all dephosphorylation reactions, is regulated by scores of regulatory proteins, including the highly conserved SDS22 protein. SDS22 has numerous diverse functions, surprisingly acting as both a PP1 inhibitor and as an activator. Here, we integrate cellular, biophysical, and crystallographic studies to address this conundrum. We discovered that SDS22 selectively binds a unique conformation of PP1 that contains a single metal (M2) at its active site, i.e., SDS22 traps metal-deficient inactive PP1. Furthermore, we showed that SDS22 dissociation is accompanied by a second metal (M1) being loaded into PP1, as free metal cannot dissociate the complex and M1-deficient mutants remain constitutively trapped by SDS22. Together, our findings reveal that M1 metal loading and loss are essential for PP1 regulation in cells, which has broad implications for PP1 maturation, activity, and holoenzyme subunit exchange.