Abstract
In eukaryotic cells, about one-third of the synthesized proteins are translocated into the endoplasmic reticulum; they are membrane or lumen resident proteins and proteins direct to the Golgi apparatus. The co-translational translocation takes place through the heterotrimeric protein-conducting channel Sec61 which is associated with the ribosome and many accessory components, such as the heterotetrameric translocon-associated protein (TRAP) complex. Recently, microscopic techniques, such as cryo-electron microscopy and cryo-electron tomography, have enabled the determination of the translocation machinery structure. However, at present, there is a lack of understanding regarding the roles of some of its components; indeed, the TRAP complex function during co-translational translocation needs to be established. In addition, TRAP may play a role during unfolded protein response, endoplasmic-reticulum-associated protein degradation and congenital disorder of glycosylation (ssr4 CDG). In this article, I describe the current understanding of the TRAP complex in the light of its possible function(s).
Highlights
In prokaryotes and eukaryotes, proteins are translocated co- and post-translationally by different pathways
Proteins and complexes that associate with the translocon (Sec61), present on endoplasmic reticulum (ER), are classified into three groups: (i) cytosolic chaperones and targeting components, such as signal recognition particle (SRP) and its receptor (SR); (ii) auxiliary components, such as translocating chain-associating membrane (TRAM), translocon-associated protein (TRAP) complex, Sec62/ 63, ERj1, binding immunoglobulin protein (BiP), calnexin and calreticulin; and (iii) modifying enzymes, such as oligosaccharyltransferase (OST)
The subunit α forms a channel via 10 transmembrane domains (TMDs)— five α-helix domains in the N-terminus and five in the C-terminus connected via a short hinge helix
Summary
Proteins are translocated co- and post-translationally by different pathways. In the conserved signal recognition particle (SRP) co-translation translocation, a similar protein-conducting channel translocates the nascent chains, Sec in eukaryotes and SecYEG in prokaryotes [1,2]. The improved resolution of cryo-electron microscopy (cryo-EM) and cryo-electron tomography (cryo-ET) has contributed considerably to the understanding of the Sec structure. The channel can open in two directions: inside (central pore) and laterally. The resident lumen proteins and proteins direct to the Golgi apparatus reach the lumen by crossing the central pore; instead, the resident membrane proteins insert in the membrane through the lateral gate (LG), a gap between two Sec TMDs that accommodates TMD α-helices [9,10,11,12]. The mature protein could play a role [19,20]; proteins that are not substrates of the translocons even by adding an SP are rejected [21,22,23]
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