SecA is a highly conserved and essential helicase-like motor protein of the bacterial Sec translocase machinery. SecA recognizes secretory proteins and couples their transport through the transmembrane SecYEG channel with the expenditure of metabolic energy provided by ATP binding and hydrolysis. Our long term goals are to (i) elucidate the regulatory properties of the helicase motor of SecA and understand how they are controlled during the ATP catalytic cycle, (ii) delineate the recognition mechanisms that enable SecA to differentiate between secretory and non-secretory proteins, and (iii) provide the structural basis of the interaction of SecA with all of its translocation ligands. The hypothesis behind the proposed research is that SecA (i) switches to a highly flexible state during its activation, wherein disorder-order transitions regulate the properties of the motor, and (ii) accomplishes the promiscuous signal sequence recognition by using alternate binding sites. Based on preliminary observations, the specific aims are designed to provide atomic-resolution insight into (i) how SecA switches among functional conformational states during the catalytic cycle, (ii) the recognition mechanisms of the promiscuous binding of signal sequences, and (iii) the interaction of SecA with preprotein substrates, the SecB chaperone and SecYEG. We aim to: 1. Delineate the conformational and dynamic properties of SecA's motor during the ATPase cycle, to understand the underlying mechanisms of the motor functionality. 2. Elucidate the molecular determinants of the promiscuous signal peptide recognition by SecA, to understand how a single translocase system is able to recognize and secrete a wide variety of protein substrates. 3. Characterize by NMR the interaction of SecA with its translocation partners (preprotein, SecB, SecYEG), to understand how the cascade of the binding phenomena results in the assembly of the complete translocase machinery. Upon bacterial infection, SecA becomes responsible for the secretion of hundreds of protein substrates, among them several toxins and adhesins that affect the human immune system. A long-term goal of the proposal is to provide the appropriate physicochemical basis towards development of antibacterial inhibitors targeting SecA.