Stimulation of hyphal growth in anaerobic cultures of Mucor rouxii by extracellular trehalose. Relevance of cell wall-bound activity of acid trehalase for trehalose utilization.

In fungi, the hydrolysis of extracellular trehalose is carried out by acid trehalases. These secretory glycoproteins may be more abundant either in the vacuolar compartment, like in yeast, or at the cell surface, such as in many filamentous fungi. The relative efficiency of these two compartments for the utilization of extracellular trehalose was investigated using as a model the dimorphic fungus Mucor rouxii, which produces yeast-like cells under a CO(2) atmosphere, or hyphae in the presence of air. Under CO(2), cultures supplemented with glucose produced yeast-like cells devoid of acid trehalase activity. On the other hand, trehalose-supplemented cultures developed hyphae exhibiting cell wall-bound and intracellular acid trehalase activity. Glucose-grown yeast-like cells supplemented with trehalose after glucose exhaustion, induced intracellular activity of acid trehalase, but no activity was detected at the cell surface. Even endowed of significant intracellular activity of acid trehalase, these cells did not grow further. When exposed to air these yeast-like produced germ tubes exhibiting cell wall-bound acid trehalase activity. These results suggest that the utilization of extracellular trehalose as a source of carbon for growth requires the localization of acid trehalase activity at the cell surface. Our results also show that extracellular trehalose elicits a morphogenetic phenomenon, inducing the formation of hyphae which are the physiological support for acid trehalase activity.

Function and regulation of the acid and neutral trehalases of Mucor rouxii.

Two different trehalose-hydrolysing activities, known as acid or non-regulatory trehalases, and neutral or regulatory trehalases, have been recognised in a number of fungal species. The true role of these apparently redundant hydrolases remained obscure for many years. However, recent evidence suggests that neutral trehalases would be specialised in the mobilisation of cytosolic trehalose, while acid trehalases would only hydrolyse extracellular trehalose. Results obtained with Mucor rouxii, a Zygomycete initially thought to possess only neutral trehalase activity, reinforced this hypothesis. M. rouxii grows efficiently in trehalose as the sole carbon source. Trehalose-grown or carbon-starved cells exhibit a high trehalase activity of optimum pH 4.5, bound to the external surface of the cell wall, in contrast with the neutral (pH 6.5) trehalase, which occurs in the cytosol. Other differences between the neutral and the acid trehalases are the temperature optimum (35 degrees C and 45 degrees C, respectively) and thermal stability (half-life of 2.5 min and 12 min at 45 degrees C, respectively). The neutral trehalase, but not the acid trehalase, is activated in vitro by cAMP-dependent phosphorylation, stimulated by Ca2+, and inhibited by EDTA. It shows maximal activity at germination and decreases as growth proceeds. In contrast the activity of the acid trehalase is totally repressed in glucose-grown cultures and increases upon exhaustion of the carbon source, and is strongly induced by extracellular trehalose.