MOLECULAR CHARACTERIZATION OF A 4TH ISOFORM OF THE CATALYTIC SUBUNIT OF PROTEIN PHOSPHATASE-2A FROM ARABIDOPSIS-THALIANA

We have recently reported the existence of multiple isoforms of the ca talytic subunit of protein phosphatase 2A (PP2A) in Arabidopsis thalia na and the molecular cloning of cDNAs encoding three of these proteins (PP2A-1, PP2A-3, PP2A-3). The reported cDNA encoding PP2A-3 was trunc ated at the 5' terminus, lacking a short fragment of the N-terminal co ding sequence. We have now isolated a near full-length cDNA encoding t he entire PP2A-3 protein (313 residues). The clone includes 188 nucleo tides of 5'-untranslated region, where a 44 bp long poly(GA) track is found. We also describe the cloning of a cDNA encoding a fourth isofor m of PP2A (PP2A-4). The polypeptide contains 313 residues being 98% id entical to PP2A-3 and only 80% identical to both PP2A-1 and PP2A-2. Th e mRNA for PP2A-4 is 1.4 kb in length and, although predominantly expr essed in roots, it is also found in other organs. It is concluded that in A. thaliana the isoforms of PP2A can be grouped in two extremely c onserved subfamilies