BACTERIAL PREPROTEIN TRANSLOCASE - MECHANISM AND CONFORMATIONAL DYNAMICS OF A PROCESSIVE ENZYME

Preprotein translocase, the membrane transporter for secretory protein s, is a processive enzyme. It comprises the membrane proteins SecYEG(D FYajC) and the peripheral ATPase SecA, which acts as a motor subunit. Translocase subunits form dynamic complexes in the lipid bilayer and b uild an aqueous conduit through which preprotein substrates are transp orted at the expense of energy. Preproteins bind to translocase and tr igger cycles of ATP binding and hydrolysis that drive a transition of SecA between two distinct conformational states. These changes are tra nsmitted to SecG and lead to inversion of its membrane topology. SecA conformational changes promote directed migration of the polymeric sub strate through the translocase, in steps of 20-30 aminoacyl residues. Translocase dissociates from the substrate only after the whole prepro tein chain length has been transported to the trans side of the membra ne, where it is fully released.