Identification and expression of NEU3, a novel human sialidase associated to the plasma membrane

Several mammalian sialidases have been described so far, suggesting the exi stence of numerous polypeptides with different tissue distributions, subcel lular localizations and substrate specificities. Among these enzymes, plasm a-membrane-associated sialidase(s) have a pivotal role in modulating the ga nglioside content of the lipid bilayer, suggesting their involvement in the complex mechanisms governing cell-surface biological functions. Here we de scribe the identification and expression of a human plasma-membrane-associa ted sialidase, NEU3, isolated starting from an expressed sequence tag (EST) clone. The cDNA for this sialidase encodes a 428-residue protein containin g a putative transmembrane helix, a YRIP (single-letter amino acid codes) m otif and three Asp boxes characteristic of sialidases. The polypeptide show s high sequence identity (78%) with the membrane-associated sialidase recen tly purified and cloned from Bos taunts. Northern blot analysis showed a wi de pattern of expression of the gene, in both adult and fetal human tissues . Transient expression in COS7 cells permitted the detection of a sialidase activity with high activity towards ganglioside substrates at a pH optimum of 3.8. Immunofluorescence staining of the transfected COS7 cells demonstr ated the protein's localization in the plasma membrane.