Paramyosin is a rod-shaped muscle protein found exclusively in invertebrates, with α-helices coiled around each other to form a coiled-coil structure. Marine organisms in which the primary structure of paramyosin has been determined are mollusks, including abalone (Haliotis discus), mussels (Mytilus galloprovincialis), octopus (Octopus bimaculoides), and oyster (Crassostrea gigas). In contrast, the primary structure of squid paramyosin, which is of particular interest, has yet to be reported. In the present study, cDNA cloning of paramyosins from four squid species, the neon flying squid (Ommastrephes bartramii), the Humboldt squid (Dosidicus gigas), the golden cuttlefish (Sepia esculenta), and the clawed armhook squid (Gonatus onyx), was performed to determine the following: the 2605-bp O. bartramii paramyosin gene containing a 2574-bp open reading frame (ORF), the 2691-bp D. gigas paramyosin gene containing a 2640-bp ORF, the 2631-bp S. esculenta paramyosin gene containing a 2574 ORF, and the 2609-bp G. onyx paramyosin gene containing a 2574-bp ORF. The primary structure of the four squid paramyosins was found to contain heptad repeats and an ACD (assembly competence domain), which are characteristic of a coiled coil. A phylogenetic analysis was performed with paramyosin sequences from species including the four squid species examined in this study, the results of which indicated that the four squid paramyosins form a group independent from the paramyosins of other species, to which octopus paramyosins are closest.
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