Lysosomal and cytosolic sialic acid 9-O-acetylesterase activities can be encoded by one gene via differential usage of a signal peptide-encoding exonat the N terminus

9-O-Acetylation is one of the most common modifications of sialic acids, an d it can affect several sialic acid-mediated recognition phenomena. We prev iously reported a cDNA encoding a lysosomal sialic acid-specific 9-O-acetyl esterase, which traverses the endoplasmic reticulum-Golgi pathway and local izes primarily to lysosomes and endosomes, In this study, we report a varia nt cDNA derived from the same gene that contains a different 5' region, Thi s cDNA has a putative open reading frame lacking a signal peptide-encoding sequence and is thus a candidate for the previously described cytosolic sia lic acid 9-O-acetylesterase activity. Epitope-tagged constructs confirm tha t the new sequence causes the protein product to be targeted to the cytosol and has esterase activity. Using reverse transcription-polymerase chain re action to distinguish the two forms of message, we show that although the l ysosomal sialic acid-specific 9-O-acetylesterase message has a widespread p attern of expression in adult mouse tissues, this cytosolic sialic acid 9-O -acetylesterase form has a rather restricted distribution, with the stronge st expression in the liver, ovary, and brain. Using a polyclonal antibody d irected against the 69-amino acid region common to both proteins, we confir med that the expression of glycosylated and nonglycosylated polypeptides oc curred in appropriate subcellular fractions of normal mouse tissues. Rodent liver polypeptides reacting to the antibody also co-purify with previously described lysosomal sialic acid esterase activity and at least a portion o f the cytosolic activity. Thus, two sialic acid 9-O-acetylesterases found i n very different subcellular compartments can be encoded by a single gene b y differential usage of a signal peptide-encoding exon at the N terminus. T he 5'-rapid amplification of cDNA ends results and the differences in tissu e-specific expression suggest that expression of these two products may be differentially regulated by independent promoters.