Structural features of the rhomboid 6TM core. The crystal structure of the 6TM core of the E. coli rhomboid protease GlpG (PDB 2NRF molecule A) is shown from three vantage points ('top view' is looking at the cell from the outside with the membrane in the plane of the page). The protein forms a compact helical bundle, with two characteristic features. A short and slanted TM4 (black) forms a helix below the catalytic serine (circled in the 'back' view), but an extended loop (L3) above it. This slanted trajectory and extended loop create a cavity above the serine. The L1 loop (purple) forms a hairpin structure that nestles between TMs 1 and 3 and protrudes laterally into the outer leaflet of the membrane (red dashed lines representing the membrane interface are provided only for reference). Catalytic dyad residues serine and histidine are in cyan; putative oxyanion-stabilizing electrophilic asparagine and histidine residues are in red.