Structural insights into tight control of PP2A methylation and function by LCMT‐1

Abstract

Protein phosphatase 2A (PP2A) is a major Ser/Thr phosphatase in all eukaryotic cells, which functions as holoenzyme consisting of catalytic (PP2Ac), regulatory and scaffolding subunits. Formation of PP2A holoenzymes is regulated by post‐translational modifications of the C‐terminus of PP2Ac (C‐tail), most notably carboxyl methylation on Leu‐309 catalyzed by leucine carboxyl methyl transferase 1 (LCMT‐1). LCMT‐1 is crucial for cell survival: its knockdown reduces the level of PP2A methylation and induces apoptosis in mammalian cells. We determined the crystal structure of the PP2Ac‐LCMT‐1 complex, trapped by covalent crosslinking of PP2A carboxyl terminus with a chemical mimic of SAM, at 2.7 Å. The structure suggests that methylation needs correct positioning of C‐tail, which, in turn, requires a direct contact of LCMT‐1 with the PP2A active site in addition to its interaction with C‐tail. To confirm structural observations, we measured dependence of PP2A methylation on the conformation of its active site. As expected, removal of catalytic Mn2+ ions led to the simultaneous elimination of PP2A phosphatase and LCMT‐1 methyltransferase activities. On contrary, stimulation of PP2A phosphatase activity by PP2A phosphatase activator (PTPA) led to stimulation of PP2A methylation. Altogether, data provide molecular basis for PP2A methylation that is tightly coupled to activation of PP2A phosphatase activity.