AMINO-ACID-SEQUENCE OF SQUID MYOSIN HEAVY-CHAIN

This work describes the determination of the cDNA sequence encoding th e myosin heavy chain (MHC) of the squid, Loligo pealei To date, the am ino-acid sequence of the MHC of calcium-regulated myosins is known onl y for two closely related species of scallops. We have determined the sequence of the entire coding region of the muscle MHC of squid, a cep halopod, and compared it with the MHC of scallops, which are pelecypod s, and to other regulated and non-regulated myosins. Residues present in the MHC of only regulated myosins have been identified. The 6504 ba se pair (bp) sequence contains an open reading frame of 5805 nucleotid es, which encodes 1935 amino acids. The sequence includes 697 bps of 3 ' untranslated sequence and 2 bps of 5' untranslated sequence. The ded uced amino-acid sequence shows the squid MHC to be 72-73% identical an d 86-87% similar to the calcium-regulated scallop MHCs cloned previous ly. In contrast, the squid MHC sequence is only 54-55% identical and 7 4% similar to skeletal MHCs of non-regulated myosins such as human fas t skeletal embryonic and human perinatal skeletal muscle, and 39-40% i dentical and 60-62% similar to smooth muscle MHC of rabbit uterus musc le and chicken gizzard muscle, respectively. We have also detected two isoforms of the MHC in squid that appear to be spliced variants of a single myosin gene. These isoforms differ in the sequence encoding the surface loop at the nucleotide binding site. Taken together, our data may help to identify more precisely the residues that are crucial in regulated myosins. (C) Chapman & Hall Ltd.