B cell receptor-associated protein alpha 4 displays rapamycin-sensitive binding directly to the catalytic subunit of protein phosphatase 2A

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 alpha 4 protein, which was discovered independently by its association with Ig-alpha in the B cell receptor complex, Herein we show that a glutathione S-transferase (GST)-alpha 4 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-alpha 4 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 alpha 4 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 alpha 4-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 alpha 4 cleaved from GST was phosphorylated by p56(lck) tyrosine kinase and protein kinase C, A FLAG-tagged alpha 4 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-alpha 4. The results reveal a novel heterodimer alpha 4-C form of PP2A that may be involved in rapamycin-sensitive signaling pathways in mammalian cells.