Core component of endosomal mRNA transport and appears to carry out crucial scaffolding functions (PubMed:31338952). The endosomal mRNA transport regulates polarity of the infectious hyphae by transporting a broad spectrum of cargo mRNAs from the nucleus to cell poles (PubMed:31338952). Might form homodimers via its C-terminal coiled-coil domain (Probable). 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 (PubMed:31338952). Interacts (via PAM2 motifs) with PAB1 (PubMed:31338952). Shuttles with RRM4-positive transport endosomes along microtubules (PubMed:31338952). Rhe endosomal localization depends on the RNA-binding capacity of RRM4 (PubMed:31338952). The PAM2 motifs are involved in the binding to PAB1. The effector domain (residues 339 to 599) is important for endosomal mRNA transport. The C-terminal coiled-coil domain might be involved in dimerization. the conserved C-terminal GWW motif is essential for endosomal localization andwhich in turn is important for the function of the protein. Causes defects in the formation of endosomal mRNPs and exhibits a bipolar growth phenotype. Belongs to the UPA1 PAM2 domain-binding protein family.