alpha 4 is a signal transduction molecule that is required for B cell activation. alpha 4 associates with the catalytic subunit of protein phosphatase 2A (PP2Ac) and regulates its enzymatic activity. We examined the interaction of alpha 4/PP2Ac with S6 kinasel (S6Kl) as a potential downstream signal transduction molecule because both alpha 4/PP2Ac association and S6Kl activity were rapamycin-sensitive. Stimulation of spleen B cells with lipopolysaccharide induced the interaction of alpha 4/PP2Ac and S6Kl. Pull-down assay demonstrated that alpha 4 interacts with S6Kl through PP2Ac. S6Kl and alpha 4 bind to the different regions of PP2Ac as S6Kl to the region from amino acid 88th to 309th of PP2Ac and alpha 4 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 alpha 4 regulates S6Kl activity through PP2Ac in B cell activation. (c) 2005 Elsevier Inc. All rights reserved.