Fig. 1. Multiple sequence alignment of Ensis directus arginine kinase (AK) domains 1 and 2 (D1 and D2, respectively), Limulus polyphemus AK and an AK consensus sequence (Ellington and Bush, 2002). Numbering is according to that of L. polyphemus AK. The 18 sequences used for the AK consensus are sea anemone Anthopleura domains 1 and 2 (O015992), crab Eriocheir (AAF43438), crab Carcinus (AAD48470), lobster Homarus (P14208), shrimp Penaeus (P51545), locust Schistocerca (P91798), bee Apis (PC6506), horseshoe crab Limulus (S52098), fruit fly Drosophila (P48610), trypanosome Trypanosoma (AAF23164), snail Turbo (O015989), abalone Suluculus (S46407), snail Cellana (BAB41096), sea hare Aplysia (BAB41095), squid Seipiateuthis (BAA95610), octopus Octopus (BAA95609) and chiton Lilophura (O15990). Catalytically important residues are shown in red. These residues contact and stabilize the reactants during catalysis. With the exception of Gly64, all the catalytically important residues are conserved in both domains of the Ensis contiguous dimer, suggesting two active domains. Residues shown in pink are those that interact with one another by forming salt bridges when the enzyme is in the closed state. The `reactive' cysteine characteristic of all phosphagen kinases is shown in green.