GlpG rhomboid serine protease (276aas ; 6 TMSs; DUF3582; Structure known) (Vinothkumar, 2011; Wang et al. 2006). The requirements for cleavage have been defined (Akiyama and Maegawa 2007). The catalytic Ser201, is located at the N terminus of TMS 4, approximately 10 Å below the membrane surface. Access to water molecules is provided by a central cavity that opens to the extracellular region and converges on Ser201. One of the two GlpG molecules in the asymmetric unit has an open conformation at the active site, with TMS 5 bent away from the rest of the molecule. Substrate entry to the active site is probably gated by the movement of TMS 5 (Wu et al. 2006). Rhomboid proteases GlpG and Rhom7 are involved in membrane protein quality control by specifically targeting components of respiratory complexes. The metastable transmembrane domains of rhomboid substrates are protected when they are incorporated into a functional complex (Liu et al. 2020).