The heterodimer of α4 and PP2Ac is associated with S6 kinase1 in B cells

α4 is a signal transduction molecule that is required for B cell activation. α4 associates with the catalytic subunit of protein phosphatase 2A (PP2Ac) and regulates its enzymatic activity. We examined the interaction of α4/PP2Ac with S6 kinase1 (S6K1) as a potential downstream signal transduction molecule because both α4/PP2Ac association and S6K1 activity were rapamycin-sensitive. Stimulation of spleen B cells with lipopolysaccharide induced the interaction of α4/PP2Ac and S6K1. Pull-down assay demonstrated that α4 interacts with S6K1 through PP2Ac. S6K1 and α4 bind to the different regions of PP2Ac as S6K1 to the region from amino acid 88th to 309th of PP2Ac and α4 to the two separated regions of the amino-terminal (from amino acid 19th to 22nd) and the middle (from 150th to 164th) portions of PP2Ac. These results suggest that α4 regulates S6K1 activity through PP2Ac in B cell activation.