The formation and activity of PP2A holoenzymes do not depend on the isoform of the catalytic subunit

Abstract

The protein phosphatase 2A holoenzyme is composed of one catalytic C subunit, one regulatory/scaffolding A subunit, and one regulatory B subunit. The core enzyme consists of A and C subunits only. The A and C subunits both exist as two closely related isoforms, α and β. The B subunits belong to four weakly related or unrelated families, designated B, B′, B″, and B‴, with multiple members in each family. The existence of two A and two C subunit isoforms permits the formation of four core enzymes, AαCα, AαCβ, AβCα, and AβCβ, and each core enzyme could in theory give rise to multiple holoenzymes. Differences between Cα and Cβ in expression and subcellular localization during early embryonic development have been reported, which imply that Cα and Cβ have different functions. To address the question of whether these differences might be caused by enzymatic differences between Cα and Cβ, we purified six holoenzymes composed of AαCα or AαCβ core enzyme and B subunits from the B, B′, or B″ families. In addition, we purified four holoenzymes composed of AβCα or AβCβ and B′ α1 or B″/PR72. The phosphatase activity of each purified form was assayed using myelin basic protein and histone H1 as substrates. We found that Cα and Cβ have identical phosphatase activities when associated with the same A and B subunits. Furthermore, no difference was found between Cα and Cβ in binding A or B subunits. These data suggest that the distinct functions of Cα and Cβ are not based on differences in enzymatic activity or subunit interaction. The implications for the relationship between the structure and function of Cα and Cβ are discussed.