Functional and structural characterization of the minimal Sec translocase of the hyperthermophile Thermotoga maritima

The genome of the hyperthermophilic bacterium Thermotoga maritima contains the genes that encode core subunits of the protein translocase, a complex consisting of the molecular motor SecA and the protein conducting pore SecYE. In addition, we identified an erroneous sequence in the genome encoding for a putative secG gene. The genes of the T. maritima translocase subunits were overexpressed in Escherichia coli and purified to homogeneity. T. maritima SecA showed a basal thermostable ATPase activity that was stimulated up to 4-fold by phospholipids with an optimum at 74°C. Membrane vesicles and proteoliposomes containing SecYE or SecYEG supported 2- to 4-fold stimulation of the precursor dependent SecA ATPase activity. Imaging of small two-dimensional crystals of the SecYE complex using electron microscopy showed square-shaped particles with a side-length of about 6 nm. These results demonstrate that in T. maritima a highly thermostable translocase complex is operational.