Post-translational modifications play an important role in regulating protein function and glycosylation is one of the most complex forms of post-translational modifications making a large contribution to phenotypic diversity. It has been established that more than 50% of proteins in eukaryotes are glycosylated1
and glycans are involved in a wide range of biological processes, such as immune response, cell–cell interaction, cellular regulation, tumor growth, and cell invasion.2-7
Glycosyltransferase (GT) is a key enzyme in the biosynthesis of glycans, which sequentially transfers a monosaccharide unit from a nucleotide sugar donor to acceptor substrates including protein residues. Understanding the function and evolution of glycosyltransferases is crucial in providing the insights into the relationships between glycan diversification, protein glycosylation and the phenotype – the main area of research in glycomics. It has been shown that knockouts of some glycosyltransferases may result in cellular dysfunction and even death.8
Moreover, in bacteria, the majority of GTs are involved in the synthesis of glycolipids, peptidoglycans, and lipopolysaccharides and can be suitable targets for drug development against bacterial pathogens. Much effort has been devoted to the identification of genes encoding glycosyltransferases and the characterization of the structures and functions of these enzymes.9,10
Currently, there are more than 100 glycosyltransferase genes found in humans and according to the CAZy database glycosyltransferases consist of approximately 90 very diverse families,11
and account for 1-2% of protein-coding genes in eukaryotic genomes (the number of genes is correlated with the number of coding genes).12
Many soluble and membrane-bound proteins form homooligomeric complexes in a cell, in fact majority of all proteins from Protein Data Bank are homooligomers. The activity of many proteins such as enzymes, ion channel proteins, receptors, and transcription factors are regulated through homooligomerization. Indeed, it has been suggested that large assemblies consisting of many identical subunits have advantageous regulatory properties as they can undergo sensitive phase transitions.13
Moreover, oligomerization can also provide sites for allosteric regulation, generate new binding sites at dimer interfaces, increase affinity through multivalent binding and enhance the diversity in the formation of regulatory complexes. The regulation of protein activity through the transitions between different oligomeric states has been experimentally demonstrated on several systems.14-17
Phosphorylation of a residue on a dimer interface or interaction with different ligands may destabilize the oligomeric structure and might shift the reaction equilibrium which might be important in regulation of apoptosis and tumor formation. In the case of low affinity interactions between proteins and glycans, it has been shown that they can be compensated through the multivalent interactions realized through homooligomerization.2
In our recent work we showed, for example, that homooligomeric interfaces in proteins from the galectin family are very well conserved among a diverse spectrum of species.18
Such homooligomeric structures allow for precise positioning of glycoligands on galectins and increases their binding affinity.
Several glycosyltransferases are similarly proven to form functional homooligomers, which play important roles in controlling enzyme activity and Golgi localization.19,20
For example, α2,6-sialyltransferase and GM2 synthase form very stable functional homodimers by disulfide bonds,21,22
In addition, a recent study indicated that glycosyltransferases may catalyze consecutive steps on the glycan biosynthesis pathways by interacting with each other and forming larger complexes.23
Indeed, there are some glycosyltransferases functioning as homooligomers while others function as monomers even though they can belong to the same protein fold class. In this paper we explore functional oligomeric states of these enzymes in various organisms and study their evolution. We also perform comparative analyses to find structural features which can mediate or disrupt the formation of biological homooligomers in glycosyltransferases. Detecting such features will help to understand the nature of regulation of enzyme activity through homooligomerization and sets the stage for prediction of functionally important oligomeric states for glycosyltransferases and other proteins.