Chaperone protein that facilitates the trafficking and functional cell surface expression of some G protein-coupled receptors (GPCRs) (PubMed:18836069). Promotes functional expression of the bitter taste receptor TAS2R16 (PubMed:16720576). Also promotes functional expression of the opioid receptor heterodimer OPRD1-OPRM1 (By similarity). In addition, acts as a potent IFN-inducible suppressor of pathogens including lyssavirus rabies, influenza A or yellow fever virus (PubMed:33113352). Mechanistically, associates with the viral replicase, binds viral RNA, and thereby suppresses viral genome amplification that replicates at the endoplasmic reticulum (By similarity). In addition, restores antiviral signaling by interacting with and sequestering influenza A virus protein NS1 (PubMed:39798334). {ECO:0000250|UniProtKB:Q9ER80, ECO:0000269|PubMed:16720576, ECO:0000269|PubMed:18836069, ECO:0000269|PubMed:33113352, ECO:0000269|PubMed:39798334}. This is the function of Ensembl gene identifier ENSG00000136514 (RTP4, receptor transporter protein 4).