Has a dual specificity toward Ser/Thr and Tyr-containing proteins. Dephosphorylates MPK4 in vitro. H2O + O-phospho-L-tyrosyl-[protein] = L-tyrosyl-[protein] + phosphate H2O + O-phospho-L-seryl-[protein] = L-seryl-[protein] + phosphate H2O + O-phospho-L-threonyl-[protein] = L-threonyl-[protein] + phosphate Inhibited by sodium vanadate and sodium tungstate. NaF and spermifine repress specifically phosphoserine and phosphothreonine phosphatase activity. Interacts with calmodulin (CaM) in a calcium Ca(2+)-dependent manner. Expressed in roots, stems, leaves and flowers. Belongs to the protein-tyrosine phosphatase family. Non-receptor class dual specificity subfamily.