Identification of kinase-phosphatase signaling modules composed of p70 S6 kinase-protein phosphatase 2A (PP2A) and p21-activated kinase-PP2A

A growing body of evidence indicates that regulation of protein-serine/thre onine phosphatase 2A (PP2A) involves its association with other cellular an d viral proteins in multiprotein complexes. PP2A-containing protein complex es may exist that contribute to PP2A's important regulatory role in many ce llular processes. To identify such protein complexes, PP2A was partially pu rified from rat brain soluble extracts following treatment with a reversibl e cross-linker to stabilize large molecular size forms of PP2A. Compared wi th native (uncross-linked) PP2A, cross-linked PP2A revealed an enrichment o f p70 S6 kinase and two pal-activated kinases (PAK1 and PAK3) in the PP2A c omplex, indicating these kinases may associate with PP2A. The existence of protein kinase-PP2A complexes in rat brain soluble ex tracts was further su bstantiated by the following results: 1) independent immunoprecipitation of the kinases revealed that PP2A co-precipitated with p70 S6 kinase and the two PAR isoforms; 2) glutathione S-transferase fusion proteins of p70 S6 ki nase and PAK3 each isolated PP2A; and 3) PAK3 and p70 S6 kinase bound to mi crocystin-Sepharose tan affinity resin for PP2A-PP1). Cumulatively, these f indings provide evidence for association of PP2A with p70 S6 kinase, PAK1, and PARS in the context of the cellular environment. Moreover, together wit h the recent reports describing associations of PP2A with Ca2+/calmodulin-d ependent protein kinase IV (Westphal, R. S., Anderson, K. A, Means, A. R., and Wadzinski, B. E. (1998) Science 280, 1258-1261) and casein kinase II al pha (Heriche, J. K, Lebrin, F., Rabilloud, T., Leroy, D., Chambaz, E. M., a nd Goldberg, Y. (1997) Science 278, 952-955), the present data provide comp elling evidence for the existence of protein kinase-PP2A signaling modules as a new paradigm for the control of various intracellular signaling cascad es.