The Molecular Biodiversity of Protein Targeting and Protein Transport Related to the Endoplasmic Reticulum

Abstract

Looking at the variety of the thousands of different polypeptides that have been focused on in the research on the endoplasmic reticulum from the last five decades taught us one humble lesson: no one size fits all. Cells use an impressive array of components to enable the safe transport of protein cargo from the cytosolic ribosomes to the endoplasmic reticulum. Safety during the transit is warranted by the interplay of cytosolic chaperones, membrane receptors, and protein translocases that together form functional networks and serve as protein targeting and translocation routes. While two targeting routes to the endoplasmic reticulum, SRP (signal recognition particle) and GET (guided entry of tail-anchored proteins), prefer targeting determinants at the N- and C-terminus of the cargo polypeptide, respectively, the recently discovered SND (SRP-independent) route seems to preferentially cater for cargos with non-generic targeting signals that are less hydrophobic or more distant from the termini. With an emphasis on targeting routes and protein translocases, we will discuss those functional networks that drive efficient protein topogenesis and shed light on their redundant and dynamic nature in health and disease.

Proteomics identifies substrates and a novel component in hSnd2-dependent ER protein targeting

“…Supporting the data from the co-immunoprecipitation and peptide spot arrays, the live cell protein-protein interaction assay demonstrated that the C-terminus of TMEM109 interacts with the N-terminus of hSnd2 (Figure 5E). In general, protein targeting receptors guide incoming polypeptides to a suitable translocase (Pool, 2022; Tirincsi et al 2022; Jomaa et al 2017 and 2021), hence crosslinking data showed that SRα binds to the Sec61 translocon (Jadhav et al 2015). Similarly, we found TMEM109 to interact strongly with the C-terminus of TRAPα and less efficiently with the C-terminus of Sec61α (Figure 5E).…”

“…So far, four protein targeting pathways which can deliver precursor polypeptides to the Sec61 protein conducting channel in the ER membrane in human cells have been described, i.e. the SRP/SR-, the SGTA/TRC/WRB-, the SND- and the PEX19/PEX3-pathway (Ast et al, 2013; Borgese et al, 2019; Tirincsi et al, 2022; Pool, 2022). While there are functional and structural insights available for the molecular mechanisms and client spectra of the SRP- and TRC-dependent pathways, little is known about the composition and mechanistic function of the human SND pathway.…”

“…The first concerns BRI3BP (see above), the second addresses the cytosolic component(s), acting upstream of the hSnd2/hSnd3 receptor, and the third relates to the possible mechanism of hSnd2/hSnd3. The four protein targeting pathways, which can deliver precursor polypeptides to the ER membrane appear to share a common principle, namely that they comprise a cytosolic component or complex, which recognises SPs and TMHs in newly synthesized ER import substrates (SRP, TRC35/Ubl4A/Bag6/TRC40, Snd1 in yeast, PEX19), plus a heterodimeric membrane receptor for the cytosolic SP- and TMH-recognition component (SRα/β, Wrb/Caml, Snd2/Snd3, PEX3+possibly PEX16) (Ast et al, 2013; Borgese et al, 2019; Tirincsi et al, 2022; Pool, 2022). Notably, however, a functional human homolog of yeast Snd1, which was shown to interact with ribosomes, has not been identified, leaving open the question as to which component(s) act(s) upstream of hSnd2/hSnd3.…”

“…Similar to the yeast Snd2 protein that shows a hydrophilic vestibule reaching into the presumed membrane environment (Liaci and Föster 2021), the predicted structure of the human hSnd2 also shows a ring of hydrophilic residues that align at the cytosolic leaflet of the ER membrane (Figure 5C). Thus, depending on the incoming precursor polypeptide, the dimeric hSnd2/hSnd3 receptor might also have an insertase function, equivalent to that of the Wrb/Caml receptor or EMC insertase, which operate through the local distortion or thinning of the membrane (Tirincsi et al 2022). Similarly, TMEM109 might represent a moonlighting protein that acts either as a cation channel upon homooligomerization or as a receptor component when associated with hSnd2 (Figure 7B).…”

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.…”