The Bilatarian Sea Urchin and the Radial Starlet Sea Anemone Globins Share Strong Homologies with Vertebrate Neuroglobins

Abstract

A 34 834-bp gene in the genome of the sea urchin Strongylocentrotus purpuratus (Echinodermata) consists of 34 exons and 33 introns, and encodes a 2252aa putative chimeric Hb, comprising an unidentifiable nonglobin N-terminal domain of ∼150aa and 16 globin domains (D1–D16) of ∼150aa, except for D2, which lacks helices G and H. The similarity between the globin domains varies from 39 to 51% identity: they are linked tightly to one another, with interdomain segments of 10aa or less. Alignment of the globin domains with other globins shows them to have a D helix and His residues at both the E7 and F8 locations. Intron insertions within the globin domains occur at canonical positions B12.2 and G7.0. Blastp searches with each of the 16 domains showed a strong sequence similarity with vertebrate neuroglobins and globin X, the Cnidarian Nematostella vectensis, and with single-domain bacterial globins. A Bayesian analysis of the S. purpuratus globins, the 7 single domain globins from the Cnidarian N. vectensis and 80 globins from several metazoan groups, indicated that the S. purpuratus and N. vectensis globins share a molecular affinity with vertebrate neuroglobins and globin X, while annelid, mollusc, crustacean, lamprey, hagfish and urochordate globins form a clade with vertebrate cytoglobins. The common molecular signatures and gene structures shared between a radial metazoan, S. purpuratus and vertebrate globins suggest these proteins could exhibit ancestral metazoan globin properties and predate the Radiata-Bilateria split. We assume modern eumetazoan globin families could have derived from such a putative globin plesiogene.