Abstract
BackgroundNeuroglobin (Ngb) is a hexacoordinated globin expressed mainly in the central and peripheral nervous system of vertebrates. Although several hypotheses have been put forward regarding the role of neuroglobin, its definite function remains uncertain. Ngb appears to have a neuro-protective role enhancing cell viability under hypoxia and other types of oxidative stress. Ngb is phylogenetically ancient and has a substitution rate nearly four times lower than that of other vertebrate globins, e.g. hemoglobin. Despite its high sequence conservation among vertebrates Ngb seems to be elusive in invertebrates.Principal FindingsWe determined candidate orthologs in invertebrates and identified a globin of the placozoan Trichoplax adhaerens that is most likely orthologous to vertebrate Ngb and confirmed the orthologous relationship of the polymeric globin of the sea urchin Strongylocentrotus purpuratus to Ngb. The putative orthologous globin genes are located next to genes orthologous to vertebrate POMT2 similarly to localization of vertebrate Ngb. The shared syntenic position of the globins from Trichoplax, the sea urchin and of vertebrate Ngb strongly suggests that they are orthologous. A search for conserved transcription factor binding sites (TFBSs) in the promoter regions of the Ngb genes of different vertebrates via phylogenetic footprinting revealed several TFBSs, which may contribute to the specific expression of Ngb, whereas a comparative analysis with myoglobin revealed several common TFBSs, suggestive of regulatory mechanisms common to globin genes.SignificanceIdentification of the placozoan and echinoderm genes orthologous to vertebrate neuroglobin strongly supports the hypothesis of the early evolutionary origin of this globin, as it shows that neuroglobin was already present in the placozoan-bilaterian last common ancestor. Computational determination of the transcription factor binding sites repertoire provides on the one hand a set of transcriptional factors that are responsible for the specific expression of the Ngb genes and on the other hand a set of factors potentially controlling expression of a couple of different globin genes.
Highlights
Globins are small heme proteins that bind various external ligands, such as oxygen or nitric oxide, and they are found in all kingdoms of life [1,2,3]
BflGb3, BflGb13 and BflGb14 represent a distinct class of globin proteins which seem to have originated through a duplication event of an ancestral globin X (GbX) gene [12,47]
Our results show that TadGb1, a putative globin of the placozoan Trichoplax adhaerens, as well as the polymeric globin from the sea urchin, are orthologous to vertebrate Ngb
Summary
Globins are small heme proteins that bind various external ligands, such as oxygen or nitric oxide, and they are found in all kingdoms of life [1,2,3]. Five recent additions to vertebrate globin repertoire are cytoglobin (Cygb), neuroglobin (Ngb), globin X (GbX), globin E (GbE) and globin Y (GbY). It has been suggested that the GbE and Mb genes, as well as the GbY and proto-Hb genes, are the paralogous products of tandem gene duplications that occurred early in the evolution of vertebrates [7,10,13,14,15]. Phylogenetic and genomic analyses indicate that the Cygb, Hb/GbY and Mb/GbE genes are products of two rounds of whole-genome duplications in the stem lineage of vertebrates [14,15]. Neuroglobin (Ngb) is a hexacoordinated globin expressed mainly in the central and peripheral nervous system of vertebrates. Despite its high sequence conservation among vertebrates Ngb seems to be elusive in invertebrates
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