UDP-GlcNAc 2-epimerase/ManNAc kinase (GNE) catalyzes the first two committed steps in sialic acid synthesis. In addition to the previously described 3 human GNE isoforms (hGNE1- hGNE3), our database and PCR analysis yielded an additional 5 human isoforms (hGNE4- hGNE8). hGNE1 is the ubiquitously expressed major isoform, while the hGNE2-8 isoforms are differentially expressed and may act as tissue-specific regulators of sialylation. hGNE2 and hGNE7 display a 31-residue N-terminal extension compared to hGNE1. Based on similarities to kinases and helicases, this extension does not seem to hinder the epimerase enzymatic active site. hGNE3 and hGNE8 contain a 55 residue N-terminal deletion, and a 50-residue N-terminal extension compared to hGNE1. The size and secondary structures of these fragments are similar, and modeling predicted that these modifications do not affect the overall fold compared to hGNE1. However, epimerase enzymatic activity of GNE3 and GNE8 is likely absent, since the deleted fragment contains important substrate binding residues in homologous bacterial epimerases. hGNE5-hGNE8 have a 53-residue deletion, which was assigned a role in substrate (UDP-GlcNAc) binding. Deletion of this fragment likely eliminates epimerase enzymatic activity. Our findings imply that GNE is subject to evolutionary mechanisms to increase cellular functions, without increasing the number of genes. Our expression and modeling data contribute to elucidation of the complex functional and regulatory mechanisms of human GNE, and may contribute to further elucidating the pathology and treatment strategies of the human GNE-opathies sialuria and hereditary inclusion body myopathy.

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