Kida et al. Even with several segments of a multispanning membrane protein already looped around inside the pore, more can be inserted.
Proteins are inserted into the ER in eukaryotes and the plasma membrane in prokaryotes by the Sec61-based translocon. Data from several studies, including crystal structures, indicate that the translocon's pore consists of a single Sec61α subunit, thus creating a narrow channel. But the new studies suggest the pore is much larger than expected.
The authors trapped intermediates in the translocation process by adding a streptavidin-binding peptide tag to the NH2-terminal end of the inserted protein. When streptavidin was added, translocation stalled, resulting in intermediates. The addition of biotin removed the streptavidin and revived translocation.
The intermediates revealed that stalled translocating peptides within the pore do not jam the translocon. Two distant hydrophilic segments of the same protein fit within the pore at the same time. As each of these lipid-averse segments needs Sec61α to protect it from the membranous environment, the structure suggests that the pore is not as narrow as previously thought. Another intermediate revealed that a stalled hydrophilic domain did not prevent the subsequent membrane insertion of as many as six successive hydrophobic segments.
To explain the flexibility, the authors suggest that perhaps two Sec61 complexes—one for each hydrophilic segment—combine for a translocation event. This model is supported by previous EM images of Sec61 oligomers. It is also possible that a single Sec61 complex undergoes drastic, unpredicted conformational changes or is somehow made larger by accessory proteins.