Membrane insertion of transmembrane proteins typically requires highly hydrophobic alpha helixes at the N-terminus, N-terminal signal peptides, tail anchors, or a combination of the three.

Byun, H., Gou, Y., Zook, A., Lozano, M., & Dudley, J. (n.d). ERAD and how viruses exploit it. Frontiers In Microbiology, 5

These processes occur co-translationally and are mediated by the SEC translocon and associated factors (especially GPI anchor transferases for tail-anchored proteins). However, my undergraduate research is focused on a viral protein which appears to be translated in association with SRP and co-translationally translocated to the ER by the translocon, but is not anchored to the membrane during translocation. This was shown (by others) by purification of the ER-associated fraction of the protein and centrifugation showing the majority pelleted with the dense, soluble fraction. A small fraction pelleted with the low density membrane-associated fraction, and my research further suggests that this membrane-associated fraction is important for its escape from the ER.

The protein I am studying functions in the cytoplasm, not in the ER or in the secretory pathway, and therefore its escape from the ER is essential for its function. The typical ERAD pathway for soluble proteins involves retrotranslocation through the Sec61 translocon channel (the same channel involved in co-translational translocation, but with different associated factors during ERAD), but this would result in unfolding of the protein prior to its retrotranslocation to the cytosol. On the other hand, the ERAD pathway for membrane proteins (the "dislocation pathway") appears to dislocate folded membrane proteins while retaining most or all of their tertiary/quaternary structure (previous citation and below).

Avci, D., & Lemberg, M. K. (2015). Clipping or Extracting: Two Ways to Membrane Protein Degradation. Trends In Cell Biology, (10), 611. doi:10.1016/j.tcb.2015.07.003

Knowing that the protein is not co-translationally inserted into the membrane because only a small fraction is membrane associated, and suspecting that the small membrane-associated fraction is important for its escape from the ER because the dislocation pathway allows folded protein to exit the ER, my question is:

How could the retrotranslocation pathway for soluble proteins be disrupted such that it would (infrequently) result in membrane insertion of this protein at an early step in retrotranslocation-coupled unfolding, such that the folded protein could follow the dislocation pathway into the cytoplasm?

I would appreciate any examples of similar processes in viruses or in eukaryotes, but any speculation on possible mechanisms - based on understanding of ERAD but lacking supporting examples - would also be greatly appreciated.

As this is unpublished research, I can't be too specific about what virus or even what model system I'm working in, but if there is additional information that would help, I'll be happy to provide it - if I can.

One final piece of information that is also relevant is that I suspect the protein's transmembrane or membrane-anchored domain is very close to its C-terminus. Another important question that this raises is whether retrotranslocation exclusively proceeds by feeding the N-terminus of a protein into the Sec61 channel, or if the C-terminus can be fed directly into the channel? If only the N-terminus could be fed into the channel, this would suggest most of the protein would have to be unfolded before the C-terminal domain became associated with the channel and was able to become membrane-associated, which voids the main benefit of the dislocation pathway; maintaining the folded state of the protein.

If somebody can answer only this question regarding the translocon (and provide some relevant reading), I would be incredibly grateful because it would suggest further literature search to help support the mechanism I am proposing.

Thank you!

  • $\begingroup$ This question is very dense and difficult to answer due to it missing key information. I've left a longer comment as an answer, but this is likely to get removed. If you could address the issues raised then a more constructive answer could be given. $\endgroup$ – James Apr 28 '16 at 7:39
  • $\begingroup$ The observed events are as follows: 1) co-translational translocation into the ER (may or may not lead to membrane insertion). This is abolished by deleting SRP and translocon components in a cellular model, and the protein is destabilized in these gene deletion mutants. 2) Low amounts of steady-state protein (but some) is membrane-bound 3) The protein may become associated with (not dissociated from) the lumenal face of the ER membrane during a failed ERAD process. The very C-terminal end of the protein probably doesn't have a transmembrane helix, the N-terminus has a 10-residue helix $\endgroup$ – Demosthenes' pars triangularis Apr 28 '16 at 14:49

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