A Membraneless Organelle Associated with the Endoplasmic Reticulum Enables 3'UTR-Mediated Protein-Protein Interactions.

Approximately half of human genes generate mRNAs with alternative 3' untranslated regions (3'UTRs). Through 3'UTR-mediated protein-protein interactions, alternative 3'UTRs enable multi-functionality of proteins with identical amino acid sequence. While studying how information on protein features is transferred from 3'UTRs to proteins, we discovered that the broadly expressed RNA-binding protein TIS11B forms a membraneless organelle, called TIS granule, that enriches membrane protein-encoding mRNAs with multiple AU-rich elements. TIS granules form a reticular meshwork intertwined with the endoplasmic reticulum (ER). The association between TIS granules and the ER creates a subcellular compartment-the TIGER domain-with a biophysically and biochemically distinct environment from the cytoplasm. This compartment promotes 3'UTR-mediated interaction of SET with membrane proteins, thus allowing increased surface expression and functional diversity of proteins, including CD47 and PD-L1. The TIGER domain is a subcellular compartment that enables formation of specific and functionally relevant protein-protein interactions that cannot be established outside.