CMP-sialic acid synthetase of the nucleus.

Sialic acids of cell surface glycoconjugates play a pivotal role in the structure and function of animal cells and in some bacterial pathogens. The pattern of cell surface sialylation is species specific, and, in the animal, highly regulated during embryonic development. A prerequisite for the synthesis of sialylated glycoconjugates is the availability of the activated sugar-nucleotide cytidine 5'-monophosphate N-acetylneuraminic acid (CMP-NeuAc), which provides the substrate for sialyltransferases. Trials to purify the enzymatic activity responsible for the synthesis of CMP-NeuAc from different animal sources demonstrated that the major localisation of the enzyme is the cell nucleus. These earlier findings were confirmed when the murine CMP-NeuAc synthetase was cloned and the subcellular transport of recombinant epitope tagged forms visualised by indirect immunofluorescence. Today, the primary sequence elements that direct murine CMP-NeuAc synthetase into the cell nucleus are known, however, information regarding the physiological relevance of the nuclear destination is still not available. With this article, we provide a detailed review on earlier and recent findings that identified and confirmed the unusual subcellular localisation of the CMP-NeuAc synthetase. In addition, we take the advantage to discuss most recent developments towards understanding structure--function relations of this enzyme.