Profiling of signal sequence characteristics and requirement of different translocation components

The N-terminal signal sequence (SS) on proteins targeted to the endoplasmic reticulum (ER) is surprisingly diverse in hydrophobicity, in the number of preceding N-terminal residues (N-length), and in charged residues flanking the sequence. However, it remains unclear how these sequences despite their heterogeneity bind to the same site and open the Sec61 translocon. We assessed varying features of SSs and their efficiencies in initiating translocation across the ER by using 5-min radiolabeling in yeast. We found that while hydrophobic SSs with a short N-length efficiently initiated translocation in Sec62 mutant, Sec63 mutant and Sec72 deletion strains, most SSs showed varying degrees of translocation defect in these strains. In particular, Sec71 was required for internal hydrophobic SSs to efficiently initiate translocation. These results suggest that different combinations of Sec62, Sec63, Sec71 and Sec72 dynamically associate with the Sec61 translocon for the optimal binding of incoming SSs of broad characteristics and the initiation of protein translocation in vivo.