Open-cap conformation of intramembrane protease GlpG

The active sites of intramembrane proteases are positioned in the lipid bilayer to facilitate pepticle bond hydrolysis in the membrane. Previous crystallographic analysis of Escherichia coli GlpG, an intramembrane protease of the rhomboid family, has revealed an internal and hydrophilic active site in an apparently closed conformation. Here we describe the crystal structure of GlpG in a more open conformation, where the capping loop L5 has been lifted, exposing the previously buried and catalytically essential Ser-201 to outside aqueous solution. A water molecule now moves into the putative oxyanion hole that is constituted of a main-chain amide (Ser-201) and two conserved side chains (His-150 and Asn-154). The loop movement also destabilizes a hydrophobic side chain (Phe-245) previously buried between transmembrane helices S2 and S5 and creates a side portal from the lipid to protease active site. These results provide insights into the conformational plasticity of GlpG to accommodate substrate binding and catalysis and into the chirality of the reaction intermediate.