(A) The structure of the monomeric E. coli SPFH protein HflK (pdb 7WI3) as well as (B) the AlphaFold (106,107) predicted structure of the Synechocystis Slr1768 protein. In (C), the structure of the E. coli HflK/C complex is shown (pdb 7WI3). (B) The protein Slr1768 of the cyanobacterium Synechocystis is predicted to have a typical HflK/C protein structure (106,107). (C,D) The E. coli HflK/C complex is partially integrated into the membrane and encloses a distinct membrane area. HflK and HflC with their transmembrane helices act like a fence with fence posts. (D) Within this E. coli complex, the FtsH protease (blue) is located. In cyanobacteria, the encoded SPFH proteins likely also form laterally enclosed, clearly defined membrane regions within the TM and/or the CM.