Can hydrolyze phosphorylated Ser-, Thr- or Tyr-substrates in vitro. The natural substrate is unknown. H2O + O-phospho-L-seryl-[protein] = L-seryl-[protein] + phosphate H2O + O-phospho-L-threonyl-[protein] = L-threonyl-[protein] + phosphate Inhibited by cadmium, copper, zinc when added cobalt when added concomitantly with manganese. Optimum pH is 6-7. Optimum temperature is 45-55 degrees Celsius. Thermostable. Neither magnesium nor calcium stimulates activity. Belongs to the PPP phosphatase family. PP-1 subfamily.