Rhomboid-type serine protease that catalyzes intramembrane proteolysis. Cleaves type-1 transmembrane domains using a catalytic dyad composed of serine and histidine that are contributed by different transmembrane domains. The loop between transmembrane domains 5 and 6 is flexible and may readily open to the extracellular side to allow water entry into the active site cavity. Belongs to the peptidase S54 family. Was originally identified as a repressor of the glycerol-3-phosphate regulon.