Investigating Translocation Mechanisms of TDP-43 Into the Endoplasmic Reticulum Using Budding Yeast Models

Signal peptides direct secretory and membrane proteins to the endoplasmic reticulum (ER), but proteins lacking classical signal peptides can occasionally engage the ER translocation machinery. TDP-43 is a nuclear RNA-binding protein implicated in amyotrophic lateral sclerosis and frontotemporal lobar degeneration. TDP-43 pathology has been linked to extracellular TDP-43 species, association with the ER luminal oxidoreductase PDI, and ER stress-related phenotypes, yet whether TDP-43 fragments can directly access the ER lumen remains unclear. Here, we used budding yeast to examine signal peptide-independent ER entry of TDP-43. C-terminal fragments of TDP-43 acquired N-glycans in ste24Δ cells, whereas full-length TDP-43 showed little detectable ER entry. Endo H digestion confirmed N-glycosylation of the fragments, and a protease protection assay supported ER luminal localization of TDP-43(Δ2-89). ER entry was reduced by sec61-41 and sec66Δ, indicating engagement of a Sec61/Sec66-dependent translocation pathway. Deletion analyses identified opposing sequence elements: residues 320-343 facilitated ER entry, whereas an N-terminal region upstream of the nuclear localization signal suppressed it. TDP-43(Δ2-89) also accessed the secretory pathway, and selected disease-associated variants caused Ire1-dependent growth vulnerability without increasing detectable ER translocation. These findings reveal a Ste24-suppressed route for noncanonical ER entry of TDP-43 fragments.