Structure of the membrane domain of an archaeal IMMP. The 6-TMS domain of the M. jannaschii enzyme, referred to as mjS2P, is shown in the open conformation with each α-helix a different color and other parts blue (PDB ID: 3B4R). The three residues that coordinate a zinc atom, which together with E55 activates a water molecule to catalyze hydrolysis of a substrate peptide bond, are shown. Part of the loop connecting TMSs 2 and 3 is predicted to enter the membrane and a 6-residue loop interrupts TMS 4. These loops are the basis for predicting analogous loops in S2P, SpoIVFB, and RseP as depicted in Figure 1.