Effects of temperature shifts on the metabolism of trehalose in Neurospora crassa wild type and a trehalase-deficient ( tre) mutant. Evidence against the participation of periplasmic trehalase in the catabolism of intracellular trehalose

Abstract

The effects of temperature shifts on the metabolism of trehalose in Neurospora crassa were studied in conidiospore germlings of a wild type strain, and of a mutant (tre), deficient in the activity of periplasmic trehalase. When the temperature of the medium was raised from 30°C to 45°C both strains accumulated trehalose, either in media supplemented with glucose or with glycerol as carbon sources. The profiles of glycolysis metabolites suggested that at 45°C glycolysis was inhibited at the level of the phosphofructokinase-1 reaction, while that of fructose-1,6-bisphosphatase was active, thus explaining how the flux of carbon from glucose or glycerol was channeled to trehalose synthesis at that temperature. This assumption was also supported by the changes in levels of fructose-2,6-bisphosphate, which dropped during the incubation at 45°C. The opposite phenomena were observed when the cultures were reincubated at 30°C and glycolysis was strongly activated. Surprisingly, the intracellular pool of trehalose of the mutant decreased after reincubation at 30°C at the same rate observed for the wild type (about 25.0 nmol/min per mg protein) despite its low trehalase activity (about 5.0 nmol/min per mg protein). Labeling experiments using [U- 14C]-glucose demonstrated that both the wild type and the mutant metabolized internally the trehalose pool, without detectable leakage of glucose or trehalose into the external medium. Cells submitted to heat shock in glycerol-supplemented medium and resuspended at 30° in the absence of an exogenous carbon source and in the presence of the glycolysis inhibitor 2-deoxyglucose accumulated high levels of free intracellular glucose, indicating that trehalose was hydrolysed internally. This suggested the existence of a cytosolic regulatory trehalase in Neurospora crassa, but all efforts to detect such activity in cell extracts have been unsuccessful so far. Altogether, these results argued against the participation of the periplasmic trehalase of N. crassa in the catabolism of intracellular trehalose. They are also conflictant with the enzyme/substrate decompartmentation hypothesis, earlier suggested as a way of explaining the mobilization of endogenous trehalose reserves accumulated in fungal spores (reviewed in Thevelein 1984, Microbiol. Rev. 48, 42–59).