Widespread Occurrence of N-Terminal Acylation in Animal Globins and Possible Origin of Respiratory Globins from a Membrane-Bound Ancestor

Proteins of the (hemo-)globin superfamily have been identified in many different animals but also occur in plants, fungi, and bacteria. Globins are renowned for their ability to store and to transport oxygen, but additional globin functions such as sensing, signaling, and detoxification have been proposed. Recently, we found that the zebrafish globin X protein is myristoylated and palmitoylated at its N-terminus. The addition of fatty acids results in an association with the cellular membranes, suggesting a previously unrecognized globin function. In this study, we show that N-terminal acylation likely occurs in globin proteins from a broad range of phyla. An N-terminal myristoylation site was identified in 90 nonredundant globins from Chlorophyta, Heterokontophyta, Cnidaria, Mollusca, Arthropoda, Nematoda, Echinodermata, Hemichordata, and Chordata (including Cephalochordata), of which 66 proteins carry an additional palmitoylation site. Bayesian phylogenetic analyses identified five major globin families, which may mirror the ancient globin diversity of the Metazoa. Globin X-like proteins form two related clades, which diverged before the radiation of the Eumetazoa. Vertebrate hemoglobin (Hb), myoglobin, cytoglobin, globin E, and globin Y form a strongly supported common clade, which is the sister group of a clade consisting of invertebrate Hbs and relatives. The N-terminally acylated globins do not form a single monophyletic group but are distributed to four distinct clades. This pattern may be either explained by multiple introduction of an N-terminal acylation site into distinct globin lineages or by the origin of animal respiratory globins from a membrane-bound ancestor. Similarly, respiratory globins were not monophyletic. This suggests that respiratory globins might have emerged independently several times and that the early metazoan globins might have been associated with a membrane and carried out a function that was related to lipid protection or signaling.