B Cell Receptor-Associated Protein α 4 Displays Rapamycin-Sensitive Binding Directly to the Catalytic Subunit of Protein Phosphatase 2A
Abstract
Recently, TAP42 was isolated as a high copy suppressor of sit4-, a yeast phosphatase related to protein phosphatase 2A (PP2A). TAP42 is related to the murine α4 protein, which was discovered independently by its association with Ig-α in the B cell receptor complex. Herein we show that a glutathione S-transferase (GST)-α4 fusion protein bound the catalytic subunit (C) of human PP2A from monomeric or multimeric preparations of PP2A in a "pull-down" assay. In an overlay assay, the GST-α4 protein bound to the phosphorylated and unphosphorylated forms of C that were separated in two-dimensional gels and immobilized on filters. The results show direct and exclusive binding of α4 to C. This is unusual because all known regulatory B subunits, or tumor virus antigens, bind stably only to the AC dimer of PP2A. The α4-C form of PP2A had an increased activity ratio compared with the AC form of PP2A when myelin basic protein phosphorylated by mitogen-activated protein kinase and phosphorylase a were used as substrates. Recombinant α4 cleaved from GST was phosphorylated by p56lck tyrosine kinase and protein kinase C. A FLAG-tagged α4 expressed in COS7 cells was recovered as a protein containing phosphoserine and coimmunoprecipitated with the C but not the A subunit of PP2A. Treatment of cells with rapamycin prevented the association of PP2A with FLAG-α4. The results reveal a novel heterodimer α4-C form of PP2A that may be involved in rapamycin-sensitive signaling pathways in mammalian cells.
- Publication:
-
Proceedings of the National Academy of Science
- Pub Date:
- September 1997
- DOI:
- 10.1073/pnas.94.20.10624
- Bibcode:
- 1997PNAS...9410624M