The multi PAM2 protein Upa2 functions as novel core component of endosomal mRNA transport.

mRNA transport determines spatiotemporal protein expression. Transport units are higher-order ribonucleoprotein complexes containing cargo mRNAs, RNA-binding proteins and accessory proteins. Endosomal mRNA transport in fungal hyphae belongs to the best-studied translocation mechanisms. Although several factors are known, additional core components are missing. Here, we describe the 232 kDa protein Upa2 containing multiple PAM2 motifs (poly[A]-binding protein [Pab1]-associated motif 2) as a novel core component. Loss of Upa2 disturbs transport of cargo mRNAs and associated Pab1. Upa2 is present on almost all transport endosomes in an mRNA-dependent manner. Surprisingly, all four PAM2 motifs are dispensable for function during unipolar hyphal growth. Instead, Upa2 harbours a novel N-terminal effector domain as important functional determinant as well as a C-terminal GWW motif for specific endosomal localisation. In essence, Upa2 meets all the criteria of a novel core component of endosomal mRNA transport and appears to carry out crucial scaffolding functions.