GLOBIN AND GLOBIN GENE STRUCTURE OF THE NERVE MYOGLOBIN OF APHRODITE ACULEATA

The globin of the nerve cord of the polychaete annelid Aphrodite acule ata 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 polypep tide chains as determined by electrospray mass spectrometry. It has an average affinity for oxygen (P-50 = 1.24 torr) resulting from fast as sociation (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 tr anslation of the cDNA sequence. The globin chain has 150 amino acid re sidues and a calculated M(r) of 15,602.69 strongly suggesting that the amino terminus is acetylated. The absence of a leader sequence and th e lack of Cys at the positions NA2 and H9 needed for the formation of the high M(r) complexes found in extracellular annelid globins classif y the Aphrodite globin with the cellular globin species. The Aphrodite nerve globin is unlikely to represent a separate globin family, as cD NA derived primers detect globin messenger RNA in muscle, gut, and pha rynx tissue as well. The gene encoding this globin species is interrup ted by a single intron, inserted at position G7.0. Comparison to other globin gene structures strongly suggest that introns can be lost inde pendently, rather than simultaneously as a result of a single conversi on event as suggested previously (Lewin, R. (1984) Science 226, 328).