Abstract
The biosynthesis of sialylated molecules of crucial relevance for eukaryotic cell life is achieved by sialyltransferases (ST) of the CAZy family GT29. These enzymes are widespread in the Deuterostoma lineages and more rarely described in Protostoma, Viridiplantae and various protist lineages raising the question of their presence in the Last eukaryotes Common Ancestor (LECA). If so, it is expected that the main enzymes associated with sialic acids metabolism are also present in protists. We conducted phylogenomic and protein sequence analyses to gain insights into the origin and ancient evolution of ST and sialic acid pathway in eukaryotes, Bacteria and Archaea. Our study uncovered the unreported occurrence of bacterial GT29 ST and evidenced the existence of 2 ST groups in the LECA, likely originating from the endosymbiotic event that generated mitochondria. Furthermore, distribution of the major actors of the sialic acid pathway in the different eukaryotic phyla indicated that these were already present in the LECA, which could also access to this essential monosaccharide either endogenously or via a sialin/sialidase uptake mechanism involving vesicles. This pathway was lost in several basal eukaryotic lineages including Archaeplastida despite the presence of two different ST groups likely assigned to other functions.
Highlights
Sialic acids are nine-carbon negatively charged monosaccharides deriving from neuraminic acid (5-amino3,5-dideoxy-D-glycero-D-galacto-2-nonulosonic acid) frequently described at terminal positions of sialylated molecules of Deuterostoma, more rarely in Protostoma
It has long been known that the oligosaccharide structure built up in the Endoplasmic Reticulum (ER) and transferred on nascent proteins in the N-glycosylation pathway is remarkably conserved in eukaryotes[78,79]
We focused on ST of the GT29 CAZy family known to catalyze the transfer of sialic acid molecules onto vertebrate glycoproteins and glycolipids
Summary
Sialic acids are nine-carbon negatively charged monosaccharides deriving from neuraminic acid (5-amino3,5-dideoxy-D-glycero-D-galacto-2-nonulosonic acid) frequently described at terminal positions of sialylated molecules of Deuterostoma, more rarely in Protostoma. 69 ST-related sequences exhibiting conserved sialylmotifs were recently identified in the Prasinophyta Bathycoccus prasinos[46] raising the possibility that these enzymes and the sialylation pathway could have appeared much sooner than anticipated in the Last Eukaryotes Common Ancestor (LECA) prior the separation of unikonts (Amorphea/opisthokonts) and bikonts (Diaphoretickes/Archaeplastida)[47]. This patchy distribution in eukaryotes raises the question of the evolutionary origin of the sialylation machinery and sialic acids. We gained strong evidences that the sialic acid metabolic pathway can be traced back to the first eukaryotes, even though many phyla have lost mandatory enzymes, like GT29 ST in Fungi and several lineages of Metazoa like worms and mollusks, or the possibility to activate the sialic acid into cytidine monophosphate sialic acid (CMP-sialic acid) in Archaeplastida
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