Analysis of the yeast homologue of neuropathy target esterase

Neuropathy Target Esterase (NTE) is an essential protein implicated in mammalian neural development and was first identified as the target for those organophosphates (OPs) that cause a delayed neuropathy. NTE has esterase activity, allowing it to hydrolyse the artificial substrate phenyl valerate in vitro: this can be followed in a simple assay. OPs inhibit this activity by reacting with the essential active site serine of NTE. Recent evidence suggests the physiological substrate of NTE to be a lysophospholipid. Previous work has shown that a region of 489 amino acids towards the C-terminal of NTE forms a domain that is sufficient for catalysis. The N-terminal portion of NTE contains regions that show similarity to cAMP-binding domains. These may contribute a regulatory role but have yet to be proved functional. On the basis of sequence data, NTE belongs to a family of proteins whose members are found in organisms ranging from bacteria to man. The previously uncharacterised protein Yml059c of the baker's yeast Saccharomyces cerevisiae belongs to the NTE family of proteins, displaying 58% similarity and 38% identity to NTE over the catalytic domain (NTE residues 727-1216) and also possessing a putative cAMP-binding motif. This study has shown that YML059c is not essential for cell viability deletion or overexpression of YML059c failed to cause any obvious phenotype. Visualisation by green fluorescent protein tagging revealed the protein to be associated with an undetermined intracellular organelle. Experiments described here show that Yml059c possesses a similar biochemical activity to mammalian NTE. Like NTE, it was able to hydrolyse phenyl valerate in an organophosphate-sensitive manner and had lysophospholipase activity. The catalytic activity of Yml059c was also dependent upon a serine residue, in an equivalent location to that of NTE's active site. In addition, Yml059c appeared able to bind radioactively labelled cAMP, suggesting a possible mechanism whereby the catalytic activity of this family of proteins may indeed be regulated by cAMP. The tentative conclusion can be drawn that Yml059c performs a similar cellular role in yeast as NTE does in mammals.