Reconsidering Redundancy

Abstract

The multimeric serine-threonine protein phosphatase PP2A, which has been implicated in signaling induced by transforming growth factor–β (TGF-β) superfamily ligands, is a heterotrimer composed of catalytic, structural, and regulatory subunits. The B family of mammalian regulatory subunits comprises four highly similar members that differ in tissue-specific and subcellular localization but that otherwise appear to be functionally redundant. Batut et al. report that two of these B subunits, Bα and Bδ, have distinct and opposite functions in mediating signaling elicited by the TGF-β superfamily ligands TGF-β, Activin, and Nodal. Bδ knockdown with morpholino antisense oligonucleotides caused an expansion of anterior structures in Xenopus laevis embryos, whereas Bα knockdown caused loss of anterior structures. The observed phenotypes, in combination with quantitative polymerase chain reaction (PCR) and in situ expression data for two positively regulated targets of Nodal signaling, suggested that Bα potentiated and Bδ inhibited Nodal signaling. In Xenopus animal cap assays, Bα knockdown blocked Activin-induced axial elongation, reduced phosphorylation of the TGF-β family signaling effector Smad2 (pSmad2), and prevented nuclear accumulation of pSmad2. In contrast, Bδ knockdown enhanced Activin-induced elongation, increased the amount of pSmad2, and increased pSmad2 nuclear accumulation, which suggested that Bα and Bδ oppositely affected Activin signaling. Similar results were seen in HeLa cells using TGF-β–induced phosphorylation and Smad2 nuclear accumulation to assay Bα and Bδ function. Both Bα and Bδ could dephosphorylate an artificial peptide and a Raf substrate in vitro, but neither dephosphorylated Smad2. Knockdown analyses in a keratinocyte cell line and in Xenopus animal caps indicated that Bα inhibits lysosomal degradation of ALK4 and ALK5, type I receptors that transduce signaling from TGF-β superfamily ligands, and that Bδ inhibits ALK4 receptor clustering. Thus, both subunits appear to influence the threshold ligand concentration that can elicit signaling but act on distinct processes to exert their opposing effects. Although the mechanism of neither subunit’s activity has been determined, it is intriguing that such similar subunits can exert opposing effects on a signaling pathway. J. Batut, B. Schmierer, J. Cao, L. A. Raftery, C. S. Hill, M. Howell, Two highly related regulatory subunits of PP2A exert opposite effects on TGF-β/Activin/Nodal signalling. Development 135 , 2927-2937 (2008). [Abstract] [Full Text]