The Arabidopsis thaliana PPX/PP4 phosphatases: molecular cloning and structural organization of the genes and immunolocalization of the proteins to plastids

The PPX/PP4 Ser/Thr protein phosphatases belong to the type 2A phosphatase subfamily and are present in most eukaryotic organisms. We have previously isolated two closely related DNAs encoding PPX isoforms (PPX-1 and PPX-2) o f Arabidopsis thaliana. Here we report the molecular cloning of the genes e ncoding these proteins. The genes PPX-1 and PPX-2 are composed of eight exo ns and seven introns located at equivalent positions related to the coding sequences. Whereas the intron-exon organization of the PPX genes is complet ely different from that of the PP2A-3/PP2A-4 A. thaliana family, specific i ntron-exon boundaries are conserved among PPX genes from distantly related organisms. Based on GUS expression, both PPX genes show the same spatial an d temporal pattern of expression: they are expressed in all the organs and tissues analyzed, and from the earliest stage of development. When PPX prot eins were localized to the root in semi-thin methacrylate sections by immun ofluorescence, staining was predominantly confined to small organelles, sho wn to be plastids by co-localization of PPX and ferredoxin. Interestingly, only some ferredoxin-positive plastids were also PPX-positive, and PPX stai ning was consistently brighter in the epidermis. The localization was confi rmed with immunogold and electron microscopy. Our results suggest that, des pite its strong sequence conservation, PPX in plants functions differently than in animals.