Akopian et al. show that the bacterial protein channel SecYEG—the homologue of eukaryotic Sec61—deforms its molecular partner to take custody of newly manufactured proteins.
SecYEG directs freshly synthesized proteins out of the cell or into the bacterial membrane. A ribosome carrying a nascent protein strand (together known as the RNC) meets up with the signal recognition particle (SRP), which then locates its receptor, FtsY, on the bacterial membrane. Together, this targeting complex delivers the nascent protein to SecYEG. Researchers didn’t know whether SecYEG was a passive recipient or took a more active role in this handoff, reshaping the targeting complex so that it drops the nascent protein.
The SRP–FtsY combo flips among several states, including an early state and an active state that unloads its cargo and allows GTP hydrolysis. The RNC stabilizes the early state, presumably preventing premature cargo release. Akopian et al. found that SecYEG undermines the early conformation but stabilizes the active form of the complex. SecYEG sticks to the ribosome and joins the targeting complex, changing its conformation to activate cargo transfer and GTP hydrolysis.
Thus, instead of receiving the nascent protein, SecYEG grabs it along with the ribosome. Phospholipids also have a role in this process. They nudge SRP–FtsY away from the early state, but they can’t overcome the countervailing effect of RNC and need help from SecYEG to dislodge the nascent protein strand.
Text by Mitch Leslie