Protein phosphatase 4 complex and its inhibitory proteins. PP4 complex is comprised of its catalytic subunit (PP4C) and regulatory subunits (PP4R1, PP4R2, PP4R3α, PP4R3β, and PP4R4). PP4C interacts with PP4R1 or PP4R4 as a heterodimer, and PP4R2 and PP4R3α/β as a heterotrimer. Target substrates of PP4 complex to be dephosphorylated are determined via regulatory subunits. The activity of PP4 complexes can be regulated by the inhibitory proteins, including PP4IP, DHX38, and TIPRL. While PP4C/PP4R2/PP4R3α complex is regulated by PP4IP or DHX38, PP4C/PP4R2/PP4R3β complex is regulated by TIPRL or DHX38. Inhibitory proteins for PP4C/PP4R1 or PP4C/PP4R4 complex have not been elucidated. The reference numbers for the information are indicated. PP4IP, PP4 inhibitory protein; DHX38, DEAH box polypeptide 38; TIPRL, Tip41-like protein.