Component of the FERRY complex (Five-subunit Endosomal Rab5 and RNA/ribosome intermediary) (PubMed:37267905, PubMed:37267906). The FERRY complex directly interacts with mRNAs and RAB5A, and functions as a RAB5A effector involved in the localization and the distribution of specific mRNAs most likely by mediating their endosomal transport. The complex recruits mRNAs and ribosomes to early endosomes through direct mRNA-interaction (PubMed:37267905). In the complex, PPP1R21 serves as a binding hub connecting all five complex subunits and mediating the binding to mRNA and early endosomes via RAB5A (PubMed:37267906). Putative regulator of protein phosphatase 1 (PP1) activity (PubMed:19389623). May play a role in the endosomal sorting process or in endosome maturation pathway (Probable) (PubMed:30520571). {ECO:0000269|PubMed:30520571, ECO:0000269|PubMed:37267905, ECO:0000269|PubMed:37267906, ECO:0000305|PubMed:19389623}. This is the function of Ensembl gene identifier ENSG00000162869 (PPP1R21, protein phosphatase 1 regulatory subunit 21).