STRUCTURAL CHARACTERIZATION OF AN ASCARIS MYOGLOBIN

Globin was purified from the body wall of adults of the parasitic nema tode Ascaris suum. Internal peptide fragments were sequenced and cDNAs encoding a polypeptide of 154 amino acids isolated by polymerase chai n reaction. The polypeptide lacks a signal sequence, identifying it as a cytosolic myoglobin-like species. The native protein is a dimer. Th e predicted amino acid sequence shares several unusual substitutions w ith other nematode globins. Like the abundant pseudocoelomic A. suum h emoglobin it has a Tyr at B10 and a Gln at E7, substitutions thought t o be determinants of high affinity. However, the 10-fold lower oxygen affinity of body wall globin suggests that in this molecule Tyr(B10) d oes not form an additional hydrogen bond with the heme bound oxygen. E volutionary analysis of the nematode globins suggests that the monodom ain myoglobin-like molecules and the two-domain hemoglobin-like molecu les diverged about 500 million years ago, well before the divergence o f the ascarid genera Ascaris and Pseudoterranova. The absence of intro ns in the A. suum myoglobin, in contrast to other nematode globin gene s, is consistent with the hypothesis that during evolution intron elim ination was the predominant event.