Functional studies of the Sec translocase: From in vitro to in vivo

In bacteria, more than one-third of the proteins synthesized in the cell accomplish their function outside the cytoplasm. For this reason, cells contain specialized transport systems that are responsible for the translocation or insertion of proteins across or into the cytoplasmic membrane. The majority of these proteins are transported via the Sec system that is universally conserved in all kingdoms of life. This system contains two main components, a protein-conducting channel across the membrane, SecYEG, and a molecular motor SecA that drives unfolded proteins through the channel. Some pathogenic bacteria contain a second SecA protein, SecA2 that is involved in the secretion of virulence factors. Our studies show that SecA1 and SecA2 interact, providing an explanation why some proteins require both SecA proteins for secretion. By using a single molecule approach in living cells, we show that the Sec system is randomly distributed across the cytoplasmic membrane as two distinct species that exhibit different membrane diffusion characteristics, likely an active and an idle complex. The work provides a deeper understanding of the mechanism of protein translocation and the molecular basis of bacterial virulence