PTEN (phosphatase and tensin homologue deleted on chromosome ten) proteins are dual phosphatases with both protein and phosphoinositide phosphatase activity. They modulate signalling pathways controlling growth, metabolism and apoptosis in animals and are implied in several human diseases. In the present paper we describe a novel class of PTEN pro-teins in plants, termed PTEN2, which comprises the AtPTEN (Arabidopsis PTEN) 2a and AtPTEN2b proteins in Arabidopsis. Both display low in vitro tyrosine phosphatase activity. In addition, AtPTEN2a actively dephosphorylates in vitro the 3′ phosphate group of PI3P (phosphatidylinositol 3-phosphate), PI(3,4)P₂ (phosphatidylinositol 3,4-bisphosphate) and PI(3,5)P₂ (phosphatidylinositol 3,5-bisphosphate). In contrast with animal PTENs, PI(3,4,5)P₃ (phosphatidylinositol 3,4,5-trisphosphate) is a poor substrate. Site-directed mutagenesis of AtPTEN2a and molecular modelling of protein–phosphoinositide interactions indicated that substitutions at the PTEN2 core catalytic site of the Lys²⁶⁷ and Gly²⁶⁸ residues found in animals, which are critical for animal PTEN activity, by Met²⁶⁷ and Ala²⁶⁸ found in the eudicot PTEN2 are responsible for changes in substrate specificity. Remarkably, the AtPTEN2a protein also displays strong binding activity for PA (phosphatidic acid), a major lipid second messenger in plants. Promoter::GUS (β-glucuronidase) fusion, transcript and protein analyses further showed the transcriptional regulation of the ubiquitously expressed AtPTEN2a and AtPTEN2b by salt and osmotic stress. The results of the present study suggest a function for this novel class of plant PTEN proteins as an effector of lipid signalling in plants. ; Anne Pribat, Rodnay Sormani, Mathieu Rousseau-Gueutin, Magdalena M. Julkowska, Christa Testerink, Jerôme Joubès, Michel Castroviejo, Michel Laguerre, Christian Meyer, Véronique Germain and Christophe Rothan