or clarity. Sec61 contains 10 TMDs that divide the protein into N- and C-terminal halves with TMD 1-5 and 6-10. Sec61 is open on one side (the front) that probably functions as a lateral gate (between TMD 2 and 7), enabling the channel to open toward the lipid bilayer. A structure of the homologous bacterial SecY channel in the closed state is provided in the bottom right panel with TMD7 in pink and TMD2 in green. One striking feature of Sec61 structure is a fully open lateral gate, while the plug is located in the center of the channel suggesting that it is closed. Opening of the lateral gate is caused by major conformational changes in TMD2, 3 and 4. Thus, opening of the gate and channel are probably separate events. Channel opening is caused by insertion of a polypeptide and subsequent displacement of the plug, whereas opening of the lateral gate is triggered by binding of a translocation partner, such as the ribosome, SecA, or Sec62/63. In the case of co-translational translocation binding of the ribosome opens the lateral gate partially, while in post-translational translocation, binding of Sec62/63 triggers wide opening of the lateral gate. This allows moderately hydrophobic signal sequences to exit into the bulk lipid and explains why these signal sequences are functional in post-translational translocation. Sec63 is associated with Sec61 via cytosolic loops 6/7 and 8/9, resulting in similar interactions as binding of the ribosome. This forces Sec61 in a gate-opened conformation. Binding of Sec63 not only controls opening of the lateral gate but also prevents ribosome binding to Sec61.
Model for protein post-translational translocation
Based on the structural data discussed above, a model for post-translational translocation has been proposed p(figure 4 adapted from 5). In the resting state (A) the channel and lateral gate are closed. The plug domain that blocks the channel is indicated by an asterisks. Binding of the Sec62/63 complex (B) primes the channel for translocation. The lateral gate is fully opened by Sec63, while the translocation channel remains blocked by the plug. A protein substrate destined for post-translational translocation is kept in an unfolded state through association with cytosolic chaperones, that include Hsp70s, and target the polypeptide to the Sec61 complex. Here, substrate proteins are captured by Sec72 that interacts with Hsp70, thereby positioning the substrate for transfer into the translocation channel. Subsequently, the polypeptide inserts as a hairpin into the channel (C) with the signal sequence (SS) exiting through the lateral gate and binds to a groove on the outside of the gate. The following polypeptide is retained in the translocation channel and at this stage chaperones can no longer remain bound because they don’t fit between Sec63 and Sec61. Therefore, all substrates are released from the polypeptide following insertion into the translocation channel. After exit of the signal sequence, the plug domain is probably displaced, which opens the channel towards the lumen. This enables translocation of the polypeptide.
It is well established that the highly conserved Sec complex plays a crucial role in protein export in all organisms (1,2). For example, in bacteria the SecY complex facilitates transport and insertion of proteins into the cytoplasmic membrane, while in eukaryotes the Sec61 channel performs the same functions in the endoplasmic membrane (2). Protein transport can occur during or after translation via the so-called post-translational or co-translational pathway. Both pathways are utilized by all organisms but their importance seems species-specific. In mammals the vast majority of proteins is directed towards the endoplasmic membrane co-translationally, while in yeast and bacteria both pathways are equally important. In eukaryotes, co-translational translocation is initiated by binding of the SRP to a hydrophobic targeting signal of the substrate protein as it emerges from the ribosome. This temporarily halts translation and the SRP-ribosome-nascent chain complex 