THE PR55 AND PR65 SUBUNITS OF PROTEIN PHOSPHATASE 2A FROM XENOPUS-LAEVIS - MOLECULAR-CLONING AND DEVELOPMENTAL REGULATION OF EXPRESSION

cDNA clones encoding the 65-kDa (PR65) and 55-kDa (PR55) regulatory su bunits of protein phosphatase 2A from Xenopus laevis were isolated by homology screening with the corresponding human cDNAs, and used to ana lyze the developmental expression patterns of these genes. The PR65 su bunit was found to be encoded by two genes, termed XPR65 alpha and XPR 65 beta. The open reading frames of the a and beta cDNAs both span 176 7 bp, and predict proteins of 64.4 kDa and 65.3 kDa, respectively, tha t are 87 % identical. The predicted amino acid sequence of XPR65 alpha showed 95 % and 84 % identity with human PR65 alpha and PR65 beta pro teins, respectively, whereas the identity of XPR65 beta with the same proteins was 87 % and 86.5 %, respectively. Only one type of Xenopus P R55 (XPR55) was isolated that showed 93 % and 84 % similarity to human PR55 alpha and PR55 beta, respectively. Analysis of the N-terminal re gion of XPR55 with the same regions of human PR55 alpha and PR55 beta, indicates that the XPR55 is the Xenopus homolog of the human PR55 alp ha isoform. Despite the overall similarity with PR55 from other specie s, XPR55 has an N-terminal extention of at least 24 amino acids. In th e ovary, a transcript of 2.8 kb, encoding the XPR65 beta, was predomin antly expressed and these XPR65 beta mRNAs are present at a constant l evel during oogenesis until late embryogenesis. Expression of the 2.4- kb XPR65 alpha was low until the larval stage, then dramatically incre ased. In all adult tissues except ovary, the 2.4-kb alpha-specific mRN B was more abundant than the 2.8-kb beta transcript. Two transcripts o f 2.4 kb and 2.5 kb, encoding the XPR55 subunit, were detected at a co nstant level throughout Xenopus oogenesis and during embryogenesis. Ba th transcripts were also expressed at similar levels in all adult tiss ues, but in a tissue-specific manner. Analysis of the XPR55 and XPR65 proteins using antibodies to recombinant proteins revealed that the ov erall levels of the two proteins were constant, in good agreement with mRNA data.