Cytosolic mannosyltransferases involved in the ER N-glycosylation pathway exhibit bacterial ancestry

N-glycosylation in eukaryotes begins with the assembly of a lipid-linked oligosaccharide on the endoplasmic reticulum membrane. As a pivotal post-translational protein modification, it is conserved across all three domains of life. However, the evolutionary origins of the N-glycosylation pathway remain a subject of ongoing debate in evolutionary biology, largely due to the limited availability of robust data regarding the evolutionary trajectories of the glycosyltransferases involved in this process. Here, we present phylogenetic analyses of the eukaryotic ALG1 and ALG2 mannosyltransferases (MTases), which are crucial for constructing the core trimannosyl Man3GlcNAc2 structure conserved in eukaryotic N-glycans. Our comprehensive phylogenetic study, combined with functional and structural analyses, suggests that the ALG2 MTase likely originated from a bacterial ancestor. This inference is further supported by the identification of sequential and functional ALG1 homologs exclusively within bacterial lineages, rather than in Asgard archaea or other archaeal groups. Our findings challenge the prevailing hypothesis that the eukaryotic N-glycosylation pathway primarily evolved from archaeal ancestors, instead suggesting a chimeric origin involving contributions from both bacterial and archaeal lineages. Synopsis O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=109 SRC="FIGDIR/small/682550v1_ufig1.gif" ALT="Figure 1"> View larger version (24K): org.highwire.dtl.DTLVardef@1ac81a8org.highwire.dtl.DTLVardef@746054org.highwire.dtl.DTLVardef@604bdborg.highwire.dtl.DTLVardef@14235d_HPS_FORMAT_FIGEXP M_FIG C_FIG The ALG glycosyltransferase (GTases) involved in the initial stage of eukaryotic N-glycosylation on the cytosolic side of the ER membrane exhibit chimeric evolutionary origins. LUCA: Last Universal Common Ancestor; LECA: Last Eukaryotic Common Ancestor. HighlightsO_LIThe evolutionary trajectories of cytosolic ALG mannosyltransferases (MTases) were studied C_LIO_LIThe ancestor of ALG2 exhibited in the LUCA but evolved its MTase activity in bacterial lineages C_LIO_LIThe ALG1 MTase originated from bacterial lineages C_LI