Examination of the twin-arginine translocation pathway in bacteria and archaea

The twin-arginine translocation (Tat) pathway is a relatively recently discovered protein translocation mechanism that is capable of facilitating the movement of prefolded proteins across cellular membranes. The overall physiological role of this pathway, as well as the mechanism of protein secretion via the Tat pathway, has been best described in Escherichia coli. However, the role and mechanism of the Tat pathway have not been described for other organisms in any great detail. This study involves the computational characterization of the twin-arginine translocation pathway in different bacteria and archaea in an effort to determine how various organisms are using the Tat pathway. Such analyses predicted that while many prokaryotes lacked the Tat pathway or used this secretory mechanism to translocate only a few substrates, other organisms targeted a significant fraction of the secretome to the Tat pathway for translocation. In addition, the wide variety of functional proteins predicted as Tat substrates indicated that in many prokaryotes the Tat pathway is a general translocation machinery (unlike what was assumed based on E. coli analyses). Two organisms with an extraordinary high number of Tat substrates, Haloferax volcanii and Streptomyces coelicolor, were further analyzed using genetic and biochemical tools. In H. volcanii , seven distinct proteins (including potential C-terminal anchored proteins and lipoproteins) were confirmed as Tat substrates. The presence of three of tat machinery genes was required for viability in H. volcanii when grown under aerobic conditions in complex media (representing the first report of requirement for this pathway under such growth conditions). The translational products of these genes also exhibited unique biochemical localization and primary structure that may represent an adaptation to the unique utilization of the Tat pathway in this organism. In our analyses of the S. coelicolor Tat pathway, it was found that many predicted Tat substrates were in fact secreted in a Tat-dependent manner, however the tat machinery genes were not essential for viability in this gram-positive bacterium