Globin and globin gene structure of the nerve myoglobin of Aphrodite aculeata

The globin of the nerve cord of the polychaete annelid Aphrodite aculeata was isolated and purified to homogeneity, The native molecule has a pi of 6.3 and acts as a dimer of two identical M(r) 15,644.5 polypeptide chains as determined by electrospray mass spectrometry. It has an average affinity for oxygen (P-50 = 1.24 torr) resulting from fast association (k(on) = 170 x 10(6) M(-1). s(-1)) and dissociation rates (k(off) = 360 s(-1)). The partial primary structure of this nerve globin was determined at the protein level and completed and confirmed by translation of the cDNA sequence. The globin chain has 150 amino acid residues and a calculated M(r) of 15,602.69 strongly suggesting that the amino terminus is acetylated. The absence of a leader sequence and the lack of Cys at the positions NA2 and H9 needed for the formation of the high M(r) complexes found in extracellular annelid globins classify the Aphrodite globin with the cellular globin species. The Aphrodite nerve globin is unlikely to represent a separate globin family, as cDNA derived primers detect globin messenger RNA in muscle, gut, and pharynx tissue as well. The gene encoding this globin species is interrupted by a single intron, inserted at position G7.0. Comparison to other globin gene structures strongly suggest that introns can be lost independently, rather than simultaneously as a result of a single conversion event as suggested previously (Lewin, R. (1984) Science 226, 328).