Protein Phosphatase 2A Regulatory Subunits

Abstract

The protein serine/threonine phosphatase 2A (PP2A) family of holoenzymes has been implicated in the regulation of many cellular signaling molecules including metabolic enzymes, cell-surface receptors, cytosohc protein kinases, and transcription factors (reviewed in refs. 1–5). PP2A is a multimeric enzyme, composed of a catalytic subunit associated with two regulatory subunits (reviewed in refs. 1–5). The core structure of PP2A is a heterodimer consisting of the 36 kDa catalytic subunit (C) tightly complexed with the 65 kDa A regulatory subunit (A). This heterodimer complexes with one of multiple B sub-units (ranging from 54–130 kDa). Currently, three B subunit families have been identified (B or B55, B′ or B56, and B″ or B72). Whereas no amino acid sequence homology exists between the different families, isoforms within the same family are highly homologous. The physiological significance of this heterogeneity is not known; however, recent data suggest that the B regulatory subunit has a role in determining the substrate selectivity of the catalytic sub-unit. It also is postulated that the B regulatory subunit participates in localization of the catalytic subunit to distinct cellular microenvironments (reviewed in refs. 1–5).