Schaper, W., Bentrop, J., Ustinova, J., Blume, L., Kats, E., Tiralongo, J., Weinhold, B., Bastmeyer, M., and Muenster-Kuehnel, A. (2012) Identification and biochemical characterization of two functional CMP-sialic acid synthetases in Danio rerio. The Journal of biological chemistry. 287(16):13239-13248.
Sialic acids (Sia) form the non-reducing end of the bulk of cell surface expressed glycoconjugates. They are, therefore, major
elements in intercellular communication processes. Addition of Sia to glycoconjugates requires metabolic activation to CMP-Sia,
catalyzed by CMP-Sia synthetase (CMAS). This highly conserved enzyme is located in the cell nucleus in all vertebrates investigated
to date, but its nuclear function remains elusive. Here, we describe the identification and characterization of two Cmas enzymes
in Danio rerio (dreCmas), one of which exclusively localized in the cytosol. We show that the two cmas genes most likely originated from the third whole genome duplication which occurred at the base of teleost radiation. cmas paralogues were maintained in fishes of the Otocephala clade, while one copy got subsequently lost in Euteleostei (e.g. rainbow
trout). In zebrafish, the two genes exhibited a distinct spatial expression pattern. The products of these genes (dreCmas1
and dreCmas2) diverged not only with respect to subcellular localization but also in substrate specificity. Nuclear dreCmas1
favored N-acetylneuraminic acid (Neu5Ac), while the cytosolic dreCmas2 showed highest affinity for 5-deamino-neuraminic acid
(KDN). The subcellular localization was confirmed for the endogenous enzymes in fractionated zebrafish lysates. Nuclear entry
of dreCmas1 was mediated by a bipartite nuclear localization signal, which seemed irrelevant for other enzymatic functions.
With the current demonstration that in zebrafish two subfunctionalized cmas paralogues co-exist, we introduce a novel and unique model to detail the roles that CMAS has in the nucleus and in the sialylation
pathways of animal cells.