Structural determinants of protein translocation in bacteria: conformational flexibility of SecA IRA1 loop region

Bacteria employ the SecA motor protein to push unfolded proteins across the cytoplasmic membrane through the SecY protein-conducting channel complex. The crystal structure of the SecA-SecY complex shows that the intramolecular regulator of ATPase1 (IRA1) SecA domain, made up of two helices and the loop between them, is partly inserted into the SecY conducting channel, with the loop between the helices as the main functional region. A computational analysis suggested that the entire IRA1 domain is structurally autonomous, and was the basis to synthesize peptide analogs of the SecA IRA1 loop region, to the aim of investigating its conformational preferences. Our study indicates that the loop region populates a predominantly flexible state, even in the presence of structuring agent. This provides indirect evidence that the SecA loop-SecY receptor docking involves loop-mediated opening of the SecY channel.