Genetic editing of SEC61, SEC62, and SEC63 abrogates human cytomegalovirus US2 expression in a signal peptide-dependent manner

Newly translated proteins enter the ER through the SEC61 complex, via either co- or post-translational translocation. In mammalian cells, few substrates of post-translational SEC62- and SEC63-dependent translocation have been described. Here, we targeted all components of the SEC61/62/63 complex by CRISPR/Cas9, creating knock-outs or mutants of the individual subunits of the complex. We show that functionality of the human cytomegalovirus protein US2, which is an unusual translocation substrate with a low-hydrophobicity signal peptide, is dependent on expression of not only SEC61α, -β, and -γ, but also SEC62 and SEC63, suggesting that US2 may be a substrate for post-translational translocation. This phenotype is specific to the US2 signal peptide.