Take Me Home, Protein Roads: Structural Insights into Signal Peptide Interactions during ER Translocation

Abstract

Cleavable endoplasmic reticulum (ER) signal peptides (SPs) and other non-cleavable signal sequences target roughly a quarter of the human proteome to the ER. These short peptides, mostly located at the N-termini of proteins, are highly diverse. For most proteins targeted to the ER, it is the interactions between the signal sequences and the various ER targeting and translocation machineries such as the signal recognition particle (SRP), the protein-conducting channel Sec61, and the signal peptidase complex (SPC) that determine the proteins’ target location and provide translocation fidelity. In this review, we follow the signal peptide into the ER and discuss the recent insights that structural biology has provided on the governing principles of those interactions.

Signal Peptide Features Determining the Substrate Specificities of Targeting and Translocation Components in Human ER Protein Import

“…Some key references are given. The lumenal or secretory proteins (Gemmer and Förster, 2020;O'Keefe et al, 2021a;Tirincsi et al, 2022a;Liaci and Förster, 2021). Typically, membrane insertion and translocation are facilitated by either a cleavable amino-terminal SP or the TMH of the nascent precursor polypeptide, which acts as a non-cleavable SP substitute.…”

“…This super-complex or Sec61 translocon can insert into the membrane or translocate into the lumen a whole variety of topologically very different precursor polypeptides (type I-, type II-, type III-, TA and hairpin membrane proteins and soluble proteins, respectively) ( Figure 3 ). Next, these precursors mature to membrane proteins with one or more hairpins or TMHs, as glycosylphosphatidylinositol- (GPI-) anchored membrane proteins, or soluble proteins in the ER lumen, such as ER-lumenal or secretory proteins ( Gemmer and Förster, 2020 ; O'Keefe et al, 2021a ; Tirincsi et al, 2022a ; Liaci and Förster, 2021 ). Typically, membrane insertion and translocation are facilitated by either a cleavable amino-terminal SP or the TMH of the nascent precursor polypeptide, which acts as a non-cleavable SP substitute.…”

“…Typically, membrane insertion and translocation are facilitated by either a cleavable amino-terminal SP or the TMH of the nascent precursor polypeptide, which acts as a non-cleavable SP substitute. Cleavable SPs are removed from the precursor polypeptides in transit by one of the two signal peptidase complexes (SPCs), which have their catalytic sites in the ER lumen ( Kalies et al, 1998 ; Chen et al, 2001 ; Liaci and Förster, 2021 ).…”

Fatty acid export (FAX) proteins contribute to oil production in the green microalga Chlamydomonas reinhardtii

“…Thus, we can conclude that Cr-FAX1 has a cleavable, N-terminal transit peptide of 54 aa and that the mature, processed Cr-FAX1 protein of 14.3 kDa is 145 amino acids long. Because signals of Cr-FAX5 antisera appeared at approximately 11–12 kDa ( Supplementary Figure S3A ), we assume that the predicted N-terminal, α-helical ER signal-sequence is actually a signal anchor sequence that is not cleaved, as observed for type II integral membrane proteins in the ER ( Liaci and Forster, 2021 ). Indeed, with 19 aa, the first hydrophobic α-helix of Cr-FAX5 is a bit too long to act as cleavable signal peptide.…”

Mechanisms of ER Protein Import

“…The next section of the Special Issue deals with the machineries for membrane insertion and translocation of proteins in the ER membrane with special emphasis on the central role of the Sec61 complex. Here, A. Tirincsi et al [8] provide up to date insights into the connections between the different targeting and translocation/insertion machineries, while P. Bhadra and V. Helms [9] as well as M. Liaci and F. Förster [10] focus on molecular dynamics and structural aspects of the Sec61 complex, respectively. This part of the Special Issue is finished off by S.-j.…”