Induction of sialic acid 9-O-acetylation by diverse gene products: Implications for the expression cloning of sialic acid O-acetyltransferases

Sialic acids can be modified by O-acetyl esters at the 7- and/or 9-position, altering recognition by antibodies, lectins and viruses. 9(7)-O-acetylation is mediated by a sialic acid- specific O-acetyltransferase, which has proven difficult to purify. Two groups have recently isolated cDNAs possibly encoding this enzyme, by expression cloning of human melanoma libraries in COS cells expressing the substrate ganglioside GD₃. Pursuing a similar approach, we have isolated additional clones that can induce 9-O-acetylation. One clone present in a melanoma library encodes a fusion protein between a bacterial tetracycline resistance gene repressor and a sequence reported to be part of the P3 plasmid. Expression of the open reading frame is necessary for inducing 9-O-acetylation, indicating that this is not a reaction to the introduction of bacterial DNA. Another clone from a rat liver cDNA library induced 9-O-acetylation on COS cells expressing α2–6-linked sialic acids, and encodes an open reading frame identical to the Vitamin D binding protein. However, truncation at the 5′ end eliminates the amino-terminal hydrophobic signal sequence, predicting cytosolic hyperexpression of a truncated protein. Thus, diverse types of cDNAs can indirectly induce sialic acid 9-O-acetylation in the COS cell system, raising the possibility that the real enzyme may be composed of multiple subunits which would not be amenable to expression cloning. Importantly, the cDNAs we isolated are highly specific in their ability to induce 9-O-acetylation either on α2–6-linked sialic acids of glycoproteins (truncated vitamin D binding protein) or on the α2–8-linked sialic acids of gangliosides (Tetr fusion protein). These data confirm our prior suggestion that a family of O-acetyltransferases with distinctive substrate specificities exists in mammalian systems.