Component of endosomal mRNA transport that regulates polarity of the infectious hyphae by transporting a broad spectrum of cargo mRNAs from the nucleus to cell poles. Part of large ribonucleoprotein complexes (mRNPs) containing RNA-binding proteins RRM4 and PAB1, endosome-binding protein UPA1, core scaffold protein UPA2 and associated factor GRP1. The endosomal localization of GRP1 depends on RRM4. In the yeast form, results in slower proliferation as well as increased cell size (PubMed:30552148). Does not cause an increased amount of bipolar cells and leads to significantly longer hyphae and empty sections at the basal pole (PubMed:30552148). Belongs to the glycine-rich RNA-binding protein family.