Human GM3 synthase: a new mRNA variant encodes an NH2-terminal extended form of the protein

All human GM₃ synthase mRNA variants until now identified predict a protein of 362 amino acids having substrate activity highly restricted to lactosylceramide. In this report we describe the identification of a new GM₃ synthase transcript containing an additional translation start codon, located upstream and in-frame with that up to now considered unique translation initiation site in the human GM₃ synthase gene. In vitro expression studies showed that the new transcript produces a longer form of human GM₃ synthase, that is efficiently translocated into the microsomal lumen and glycosylated. Moreover, stable cDNA transfection into mammalian cells gives rise to a threefold increase of GM₃ synthase activity, associated to a broader substrate specificity. Although this transcript has been initially identified in the human placenta, RT-PCR analyses verified the expression of an identical mRNA also in undifferentiated HL60 cells, but not in the monocytic lineage. Altogether, these results are the first demonstration of the existence of a new isoform of human GM₃ synthase, which could play an important role during HL60 cell differentiation. The functional relevance of the existence of two isoforms of GM₃ synthase is also discussed.