Endosomal hitchhiking and NDR kinase signaling coordinate SsdA-mRNP localization

Transport and regulation of messenger ribonucleoprotein complexes (mRNPs) is critical for spatial control of gene expression within cells. How mRNPs are trafficked in hyphae of different filamentous fungi remains poorly understood. Here we investigate the transport of SsdA, the Aspergillus nidulans ortholog of budding yeast RNA-binding protein Ssd1, which is thought to function in translational repression. SsdA forms cytoplasmic puncta that colocalize with the poly(A)-binding protein FabM, marking them as mRNPs, and punctum formation depends on conserved RNA-binding residues of SsdA. SsdA puncta move bidirectionally along microtubules by hitchhiking on early endosomes, and this requires the adaptor proteins PxdA and DipA--machinery previously associated exclusively with peroxisome transport. SsdA puncta are largely depleted from hyphal tip-proximal regions, despite an abundance of early endosomes near hyphal tips, and the length of SsdA puncta depletion zones correlates with hyphal growth rate. We show that acute inhibition of Nuclear Dbf2-related (NDR)-family protein kinase CotA causes rapid accumulation of SsdA puncta near hyphal tips, indicating that puncta depletion depends on CotA activity. Mutation of predicted CotA phosphorylation sites within SsdA to nonphosphorylatable residues also leads to accumulation of SsdA puncta near tips, while mutation to phosphomimetic residues disrupts puncta entirely. Together, our findings establish SsdA-containing mRNPs as a new endosomal hitchhiking cargo in A. nidulans and reveal that NDR kinase signaling spatially regulates SsdA-mRNP distribution during normal polarized growth.