| Summary: | PP2A (protein serine/threonine phosphatase 2A), a conserved protein found in different species, was involved in various cellular processes including cell cycle, signal transduction, DNA replication, transcription, protein synthesis and apoptosis. Deregulation of PP2A methylation is linked to Alzheimer's disease and increased susceptibility to pathogen infection. PP2A was identified as to be an important tumour suppressor protein and hence a potential target for cancer therapeutic strategies. PP2A comprises a core structure of a 65 kDa scaffolding subunit A (PP2AA) and a 36 kDa catalytic subunit C (PP2AC), which associates with a variable regulatory subunit B to form a heterotrimeric holoenzyme. Different heterotrimeric compositions influence the enzyme's cellular location and substrate specificity. Leucine carboxyl methyltransferase 1 (LCMT1) and protein phosphatase methylesterase 1 (PME1) are involved in PP2A's methylation and de-methylation respectively. Recent research has demonstrated that methylation of PP2A subunit C at the carboxyl-terminus, residue Leu309, by LCMT1 is related to the selection of the regulatory subunit to form a triple complex with the PP2A AC core enzyme (PP2AD). In contrast, PME1 is thought to associate with and stabilize the inactivated form of PP2A complex. The crystallographic structure of PME1-PP2A complex was solved in 2008 and was suggested that PME1 only associates with the catalytic subunit of PP2A; however, both PME1 and PP2A scaffolding subunit are not native form. In this study, we used isothermal titration calorimetry (ITC) and surface plasmon resonance (SPR) methods to verify the association of PME1 and PP2AA and measured thermodynamic parameters of PME1-PP2AA interaction. Finally, we have solved an X-ray crystal structure of human LCMT1 protein in complex with the co-factor S-adenosylmethionine (AdoMet) to resolution of 2 Å. The structure enables us to postulate a mode of interaction with protein phosphatase PP2A and it provides a platform for further functional studies of the methylation regulation of PP2A.
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