Figure 1.

Structure of the PSPs. PP1 is comprised of catalytic and regulatory subunits. It can be inhibited by regulatory subunit-specific inhibitors. PP2A is comprised of catalytic, regulatory and scaffold subunits. Activators can stabilize the holoenzyme. Calcineurin is comprised of catalytic and regulatory subunits, which can be inhibited by inhibitors such as CSP and FK506. PP4 can form the dimer with PP4R1 or trimer with PP4R2/PP4R3. PP5 functions as a single subunit enzyme. It features a tetratricopeptide repeat domain and an autoinhibitory sequence. PPM1D is a single-unit enzyme with the B-loop facing the catalytic center. Four Asp residues facilitate the binding of metal iron. Asp: aspartic Acid, CSP: cyclosporin