Both YidC and SecYEG Are Required for Translocation of the Periplasmic Loops 1 and 2 of the Multispanning Membrane Protein TatC
TatC, a subunit of the twin arginine translocase, is a 6-membrane-spanning protein exposing three periplasmic loops. We have used TatC as a model system to examine how multispanning proteins insert into the membrane. To assay translocation of each of the three loops of TatC across the membrane, we used trypsin mapping, proteinase K mapping, and chemical modification methods. Here, we show that the signal recognition particle is required for targeting TatC to the inner membrane of Escherichiacoli. While translocation of loops 1 and 2 is strictly dependent on the Sec translocase and the YidC insertase, translocation of loop 3 does not depend on the translocase or insertase. None of the periplasmic loops require SecA or the proton motive force for membrane translocation. This work demonstrates a strategy where all the loops of a multispanning membrane protein can be monitored individually. The membrane translocation mechanism of each periplasmic loop can be complex with different energy and translocase requirements for a multispanning membrane protein. âº Individual loops of the multispanning TatC are translocated by different mechanisms. âº YidC functions to clear the SecYEG channel of inserting membrane proteins. âº Translocation of protein regions of membrane proteins can be monitored by various proteases and alkylating agents.