In bacteria, the majority of exported proteins are translocated by the Sec system, which recognizes the signal sequence of
a preprotein and uses ATP and the proton motive force to mediate protein
translocation across the cytoplasmic membrane. SecA is an essential protein component of this system, containing the
molecular motor that facilitates translocation. Here we report the three-dimensional structure of the SecA protein of Mycobacterium tuberculosis. Each subunit of the homodimer contains a "motor" domain and a translocation domain. The structure predicts that SecA can interact with the SecYEG pore and function as a molecular ratchet that uses ATP hydrolysis for physical movement of the preprotein. Knowledge of this structure provides a framework for further elucidation of the translocation process.