Protein phosphatase 1 regulatory subunit 21 (PPP1R21) is a regulator of protein phosphatase 1 (PP1) activity [2] and may play a role in the endosomal sorting process or in endosome maturation pathway [1]. This entry represents a middle domain found between the N- and C-terminal coiled-coils. According to structure predictions, it appears to form a six-helix bundle.

1: Biallelic loss of function variants in PPP1R21 cause a neurodevelopmental syndrome with impaired endocytic function. Rehman AU, Najafi M, Kambouris M, Al-Gazali L, Makrythanasis P, Rad A, Maroofian R, Rajab A, Stark Z, Hunter JV, Bakey Z, Tokita MJ, He W, Vetrini F, Petersen A, Santoni FA, Hamamy H, Wu K, Al-Jasmi F, Helmstadter M, Arnold SJ, Xia F, Richmond C, Liu P, Karimiani EG, Karami Madani G, Lunke S, El-Shanti H, Eng CM, Antonarakis SE, Hertecant J, Walkiewicz M, Yang Y, Schmidts M; Hum Mutat. 2019;40:267-280. PMID:30520571

2: Docking motif-guided mapping of the interactome of protein phosphatase-1. Hendrickx A, Beullens M, Ceulemans H, Den Abt T, Van Eynde A, Nicolaescu E, Lesage B, Bollen M; Chem Biol. 2009;16:365-371. PMID:19389623