Loopy Sec61 mutants

Sec61 forms the pore through which proteins enter the ER. On page 67, Cheng et al. show that its cytoplasmic face makes at least two contributions to translocation before the protein passage event.The predicted structure of yeast Sec61 suggests that several conserved charged residues, which are in cytoplasmic loops that link adjacent membrane-spanning domains, may interact with the ribosome during cotranslational translocation. The authors mutated many of these candidate residues within Sec61. Several of the mutations blocked cotranslational protein translocation in vivo, but the particular effect depended on which loop was mutated.

Point mutations in loop 8 (L8) interfered with the binding affinity between the ribosomal large subunit and the pore. Without this contact, normally ER-localized proteins accumulated in the cytoplasm. Cytoplasmic protein accumulation was also seen when...