The structural basis for tight control of PP2A methylation and function by LCMT-1

Area of Science:

• Biochemistry
• Molecular Biology
• Cell Biology

Background:

• Proper formation of protein phosphatase 2A (PP2A) holoenzymes is crucial for eukaryotic cell fitness.
• Carboxyl methylation of the PP2A catalytic subunit is critical for holoenzyme assembly.
• This methylation is catalyzed by the PP2A-specific methyltransferase, Leucine-rich repeats and coiled-coil domain containing protein 1 (LCMT-1), which is essential for cell survival.

Purpose of the Study:

• To determine the crystal structures of human LCMT-1 in isolation and in complex with PP2A.
• To elucidate the mechanism by which LCMT-1 regulates PP2A holoenzyme formation and function.
• To understand the role of LCMT-1 in controlling PP2A activity for cell cycle progression and survival.

Main Methods:

• X-ray crystallography to determine the structures of LCMT-1 and its complex with PP2A.
• Biochemical assays to demonstrate the functional consequences of PP2A active site activation on methylation.
• Analysis of a dominant-negative LCMT-1 mutant to assess its impact on the cell cycle.

Main Results:

• Crystal structures revealed that LCMT-1's active site recognizes the carboxyl terminus of PP2A.
• Extensive contacts between the PP2A active site and LCMT-1 were observed.
• Activation of the PP2A active site was shown to stimulate LCMT-1-mediated methylation.
• A dominant-negative LCMT-1 mutant caused cell cycle attenuation without cell death.

Conclusions:

• LCMT-1 tightly controls PP2A function through its interaction with the PP2A active site.
• This interaction ensures efficient conversion of activated PP2A into substrate-specific holoenzymes, preventing unregulated phosphatase activity.
• LCMT-1's regulatory mechanism is vital for maintaining cell cycle progression and cell survival.