2.A.6.4.1
The secretory accessory proteins, SecDF. The first periplasmic domain of SecDF has been crystallized (Echizen et al., 2011) as has the intact SecDF complex (Tsukazaki and Nureki 2011). SecDF has been reported to function as a pmf-driven H⁺ transporter that facilitates protein translocation (Tsukazaki et al. 2011).  It may assume at least two conformations differing by a 120 degrees rotation during polypeptide translocation (Mio et al. 2014). SecDF is proposed to undergo repeated conformational transitions to pull out the precursor protein from the SecYEG channel into the periplasm (Tsukazaki 2018). Once SecDF captures the precursor protein on the periplasmic surface, it can complete protein translocation even if SecA function is inactivated by ATP depletion, implying that SecDF is a protein-translocation motor that works independent of SecA. Structural and functional analyses of SecDF suggested that SecDF utilizes the proton gradient and interacts with precursor proteins in the flexible periplasmic region. The crystal structures of SecDF in different states at more than 3 Å resolution were reported in 2017 and 2018, which further improved our understanding of the dynamic molecular mechanisms of SecDF (Tsukazaki 2018).