Ever Surprising Nematode Globins
Nematodes express pseudocoelomic, body wall and cuticle globin isoforms. All globin isoforms display the major determinants of the globin fold and a B10Tyr/E7Gln residue pair, which is a signature of high oxygen affinity. The hitherto studied pseudocoelomic globins are octamers of covalently linked didomain globin chains. Body wall globins so far are monomeric, whereas cuticle globins are tetrameric. The extremely high oxygen affinity of the pseudocoelomic globins is caused by a network of three H-bonds between the bound ligand, B10Tyr and E7Gln resulting in a very low dissociation rate. The body wall and cuticle globins, albeit also displaying B10Tyr and E7Gln, have more moderate oxygen affinities. The structural reason for the latter observation is unknown. Although many hypotheses have been put forward, the real function of the nematode globins remains illusive.
Caenorhabditis elegans expresses 33 globin-like proteins. They display the major determinants of the globin fold and are expressed at very low levels. Most of them have N- and C-terminal extensions as well as interhelical insertions of variable length. Orthologues of these globins have been identified in closely related species and also in other nematode taxa.
Introns inserted at B12.2 and G7.0 are common in nematode globin genes and the E-helix is also interrupted by an intron, however at more variable positions. The globins of C. elegans are unique in having more introns that seem to be inserted rather randomly. Thus the intron insertion pattern of the nematode globin introns substantially deviates from the conserved intron/exon pattern seen in vertebrates.
Phylogenetic analysis of all nematode globin sequences reveals two strictly separated clades, one comprises all C. elegans globins except ZK637.13 and the other groups ZK637.13 and all other nematode globins. This might suggest that the globins in both clades have acquired different functions.
KeywordsBody Wall Parasitic Nematode Globin Gene Intron Gain Intron Insertion
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