In situ structures of secretins from bacterial type II secretion system reveal their membrane interactions and translocation process.

The GspD secretin is the outer membrane channel of the bacterial type II secretion system (T2SS) which secrets diverse effector proteins or toxins that cause severe diseases such as diarrhea and cholera. GspD needs to translocate from the inner to the outer membrane to exert its function, and this process is an essential step for T2SS to assemble. Here, we investigate two types of secretins discovered so far in Escherichia coli , GspD α and GspD β , respectively. By electron cryotomography subtomogram averaging, we determine in situ structures of all the key intermediate states of GspD α and GspD β in the translocation process, with resolution ranging from 9 Å to 19 Å. In our results, GspD α and GspD β present entirely different membrane interaction patterns and ways of going across the peptidoglycan layer. We propose two distinct models for the membrane translocation of GspD α and GspD β , providing a comprehensive perspective on the inner to outer membrane biogenesis of T2SS secretins.