Probe Development Efforts to Identify Novel Inhibitors of Protein Phosphatase Methylesterase-1 (PME-1)

Reversible protein phosphorylation networks play essential roles in most cellular processes. While over 500 kinases catalyze protein phosphorylation, only two enzymes, PP1 and PP2A, are responsible for more than 90% of all serine/threonine phosphatase activity. Phosphatases, unlike kinases, achieve substrate specificity through complex subunit assembly and post-translational modifications rather than number. Mutations in several of the PP2A subunits have been identified in human cancers, suggesting that PP2A may act as a tumor suppressor. Adding further complexity, several residues of the catalytic subunit of PP2A can be reversibly phosphorylated, and the C-terminal leucine residue can be reversibly methylated. Protein phosphatase methylesterase-1 (PME-1) is specifically responsible for demethylation of the carboxyl terminus. Methylesterification is thought to control the binding of different subunits to PP2A, but little is known about physiological significance of this post-translational modification . Recently, PME-1 has been identified as a protector of sustained ERK pathway activity in malignant gliomas. PME-1 knockout mice generated by targeted gene disruption result in perinatal lethality, underscoring the importance of PME-1 but hindering biological studies. The Scripps Research Institute Molecular Screening Center (SRIMSC), part of the Molecular Libraries Probe Production Centers Network (MLPCN), identified a potent and selective PME-1 inhibitor probe, ML174, by high-throughput screening using fluorescence polarization-activity-based protein profiling (FluoPol-ABPP). ML174, with an IC of 10 nM, is based on the aza-beta-lactam scaffold and is selective for PME-1 among serine hydrolases in human cell line proteomes as assessed by gel-based competitive-activity-based protein profiling. Among more than 30 serine hydrolase anti-targets, ML174 is selective at 1 μM. Additionally, ML174 was shown to be highly active against PME-1 and to result in 85% reduction of demethylated PP2A. We previously reported a modestly potent 500 nM inhibitor that was selective for PME-1, the first reported selective PME-1 inhibitor. ML174 is 50 times more potent and from an entirely different structural and mechanistic class of inhibitors. Due to its much higher potency, ML174 has greater potential for use in long time-course studies, and is a much better candidate for applications.