Analysis of subunit isoforms in protein phosphatase 2A holoenzymes from rabbit and Xenopus

A dimeric and two trimeric forms of protein phosphatase 2A (PP2A) were purified from rabbit and Xenopus tissues and analyzed using antisera specific for the catalytic and regulatory subunits. The dimeric holoenzyme consists of a complex between a 36-kDa catalytic subunit associated with a approximately 65-kDa regulatory subunit. The two trimeric holoenzymes consist of the catalytic subunit complexed with 65- and 55-kDa subunits, or 65- and 72-kDa subunits. Antisera were raised against synthetic peptides specific for the alpha- and beta-isoforms of the 65-kDa (PR65 alpha/beta) and 55-kDa (PR55 alpha/beta) subunits identified by molecular cloning. Anti-peptide antisera to the 36-kDa catalytic subunit of PP2A were prepared against two selected regions: one specific for the alpha-isoform and one to a peptide common to both the alpha- and beta-isoforms. Immunochemical analysis of all three mammalian holoenzymes showed that the catalytic, 55- and 65-kDa subunits are both predominantly of the alpha-isoform, which is consistent with the peptide sequence data. The 65-kDa subunit of PP2A holoenzymes isolated from Xenopus skeletal muscle reacted with both anti-alpha and anti-beta PR65-specific antisera whereas the PP2A holoenzymes isolated from Xenopus oocytes reacted preferentially with the beta-specific antisera, indicating developmental changes in the expression of the 65-kDa subunit isoform. Taken together, these results show that the "core" subunits of the PP2A holoenzymes consist of the catalytic complexed with the 65-kDa subunit and that the association of the third subunit does not appear to be influenced by the isoform of these two core subunits.