The Enigma of NTH2 Gene in Yeasts

The enzymatic hydrolysis of the non-reducing disaccharide trehalose in yeasts is carried out by trehalase, a highly specific α-glucosidase. Two types of such trehalase activity are present in yeasts, and are referred to as neutral and acid enzymes. They are encoded by distinct genes (NTH1 and ATH1, respectively) and exhibit strong differences in their biochemical and physiological properties as well as different subcellular location and regulatory mechanisms. Whereas a single gene ATH1 codes for acid trehalase, the genome of some yeasts appears to predict the existence of a second redundant neutral trehalase, encoded by the NTH2 gene, a paralog of NTH1. In S. cerevisiae the corresponding two proteins share 77% amino acid identity, leading to the suggestion that NTH2 codes for a functional trehalase activity. However, Nth2p lacks any measurable neutral trehalase activity and disruption of NTH2 gene has no effect on this activity compared to a parental strain. Likewise, single nth1Δ and double nth1Δ/nth2Δ null mutants display no detectable neutral activity. Furthermore, disruption of NTH2 does not cause any apparent phenotype apart from a slight involvement in thermotolerance. To date, no evidence of a duplicated NTH gene has been recorded in other archetypical yeasts, like C. albicans or C. parapsilosis, and a possible regulatory mechanism of Nth2p remains unknown. Therefore, although genomic analysis points to the existence, in some yeasts, of two distinct genes encoding trehalase activities, the large body of biochemical and physiological evidence gathered from NTH2 gene does not support this proposal. Indeed, much more experimental evidence would be necessary to firmly validate this hypothesis.