Protein Ser/Thr phosphatases with kelch-like repeat domains.

This report describes the presence in plants of protein Ser/Thr phosphatases of the PPP family, homologous to PfPPalpha phosphatase from Plasmodium falciparum. Like PfPPalpha, they possess large N-terminal domains and catalytic domains that are more closely related to the protein phosphatase 1 group. The N-terminal domains of PfPPalpha and its plant homologues contain tandem kelch-like repeats, not previously identified in any protein phosphatases, suggesting that the N-terminal domains may form beta-propeller structures mediating protein-protein interactions. We therefore suggest that this novel phosphatase group be designated as PPKLs for protein phosphatases with kelch-like repeat domains. Four PPKL isoforms are encoded in the Arabidopsis thaliana genome, of which at least three are expressed. PPKLs appear to be ubiquitous in Viridiplantae. The existence of a protein phosphatase group shared by Viridiplantae and Apicomplexa, but not other eukaryotes, is in line with the theory of the origin of Apicomplexa by endosymbiosis of nonphotosynthetic eukaryotes with red algae.