Substrate Proteins Take Shape at an Improved Bacterial Translocon.

Characterization of Sec-dependent bacterial protein transport has often relied on an in vitro protein translocation system comprised in part of E. coli inverted inner membrane vesicles or more recently purified SecYEG translocons reconstituted into liposomes using mostly a single substrate (proOmpA). A paper published in this issue of the Journal of Bacteriology (P. Bariya and L. Randall, J Bacteriol, this issue) finds that inclusion of SecA protein during SecYEG proteoliposome reconstitution dramatically improves the number of active translocons: both an experimentally useful and intriguing result that may arise from SecA membrane integration properties as discussed here. Furthermore, determination of the rate limiting transport step for nine different substrates implicates the mature region distal to the signal peptide in the observed rate constant differences, indicating that more nuanced transport models that respond to differences in protein sequence and structure are needed.