Membrane translocation process revealed by in situ structures of type II secretion system secretins

The GspD secretin is the outer membrane channel of the bacterial type II secretion system (T2SS) which secrets diverse 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. By electron cryotomography subtomogram averaging, we determine in situ structures of 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 transitioning the peptidoglycan layer. From this, we hypothesize 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.