Abstract
Protein N-glycosylation (PNG) is crucial for protein folding and enzymatic activities, and has remarkable diversity among eukaryotic species. Little is known of how unique PNG mechanisms arose and evolved in eukaryotes. Here we demonstrate a picture of onset and evolution of PNG components in Golgi apparatus that shaped diversity of eukaryotic protein N-glycan structures, with an emphasis on roles that domain emergence and combination played on PNG evolution. 23 domains were identified from 24 known PNG genes, most of which could be classified into a single clan, indicating a single evolutionary source for the majority of the genes. From 153 species, 4491 sequences containing the domains were retrieved, based on which we analyzed distribution of domains among eukaryotic species. Two domains in GnTV are restricted to specific eukaryotic domains, while 10 domains distribute not only in species where certain unique PNG reactions occur and thus genes harboring these domains are supoosed to be present, but in other ehkaryotic lineages. Notably, two domains harbored by β-1,3 galactosyltransferase, an essential enzyme in forming plant-specific Lea structure, were present in separated genes in fungi and animals, suggesting its emergence as a result of domain shuffling.
Highlights
In this article, we investigate how genes responsible for protein N-glycosylation (PNG) arose in eukaryotes
The glycan core is usually substituted by a β1,2 xylose, which is catalyzed by β1,2 xylosyltransferase (β1,2-XylT), and the proximal N-acetylglucosamine is replaced by an α1,3-fucose through catalysis of α1,3-fucosyltransferase (α1,3-FucT); in higher plants a typical N-glycan usually contains a Lewis a (Lea) structure, which is formed by attachment of β1,3 galactose and α1,4 fucose to the terminal GlcNAc, facilitated by β1,3-GalT and α1,4-FucT, respectively (Fig. 1b)
Domains in 15 out of 22 glycosyltransferase genes in PNG could be classified into the clan CL0110, suggesting that these genes have risen from a single evolutionary origin[23]
Summary
We investigate how genes responsible for protein N-glycosylation (PNG) arose in eukaryotes. A single α1,6 mannose unit is first attached to the glycan by Och[1]; it is elongated by multi-enzyme complexes, M-PolI and M-PolII, to form the α1,6 outer chain backbone containing up to 50 additional mannose residues It is further decorated with side chains mainly consisting of homopolymeric α1,2 mannosides and heteropolymeric α1,2/α1,3 or α1,2/β1,2 mannosides, catalyzed by Mnn[1], Mnn[2], Mnn[5] and Mnn[6], respectively[17,18]. We aim to systematically investigate how molecular mechanisms of PNG in Golgi emerged and evolved, based on which we propose how they shaped diversity and novelty of protein N-glycans in different eukaryotic lineages. Based on the gene sequences retrieved, we sought to answer when novel mechanisms of PNG possibly occurred, and how they evolved in fungi, animals and plants
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