Plays a key role in signaling protein misfolding via the CpxR/CPXA transducing system. It also modulates the phosphorylated status of many phosphoproteins in E.coli, some of which acting as major chaperones. Has been shown, in vitro, to act on Ser, Thr and Tyr-phosphorylated substrates. H2O + O-phospho-L-seryl-[protein] = L-seryl-[protein] + phosphate H2O + O-phospho-L-threonyl-[protein] = L-threonyl-[protein] + phosphate Binds 2 manganese ions per subunit. Belongs to the PPP phosphatase family. PP-1 subfamily.