Er Translocation Landscape: From Molecular Mechanism To Drug Development

The translocation of newly synthesised polypeptides into the endoplasmic reticulum (ER) is driven by specific signals that direct their insertion and passage through the conserved heterotrimeric Sec61 channel. The functional effectiveness of these signals depends on their amino acid composition and precise location within the polypeptide, which in turn governs the kinetics of ER entry and distinguishes co-translational from post-translational translocation pathways. While strong N-terminal signals have long been associated with immediate, co-translational ER targeting, emerging evidence indicates that weaker or marginally hydrophobic N-terminal sequences can introduce a controlled delay in translocation initiation. We show that such delayed translocation exposes nascent polypeptide chains transiently to the cytosol, thereby enabling novel co- and pre-translocational modifications, such as cryptic-site phosphorylation, with significant effects on protein folding, intracellular trafficking, and overall stability. These insights uncover an additional layer of regulatory complexity in protein biogenesis and ER targeting. Moreover, in the last decade, the tight interdependence between translation and translocation, coupled with the uniqueness of translocation signals, has proven valuable for drug development through the generation of selective Sec61 inhibitors. In this context, we have recently developed a sensitive and rapid luciferase-based reporter assay to facilitate the screening of Sec61 inhibitors capable of selectively down-modulating proteins implicated in diverse human diseases, including cancer and inherited disorders. In conclusion, these results offer new perspectives on the regulation of protein biosynthesis, suggesting that a deeper understanding of translocation signals could pave the way for the development of innovative therapeutic strategies for the treatment of numerous human diseases.