Excretion of two proteases by the ectomycorrhizal fungus Amanita muscaria

Abstract

In the soils of mature forests, nitrogen availability is mainly the result of litter decomposition. Thus protein degradation is of major interest for nutrition. Two aspartic proteases were excreted in a pH-dependent manner by the ectomycorrhizal fungus Amanita muscaria grown in liquid culture. AmProt1 with a molecular mass of approximately 45 kDa was mainly present at pH values up to 5.4, whereas the excretion of AmProt2 with a molecular mass of about 90 kDa was only detectable at pH values between pH 5.4 and 6.3. In addition, the pH optima of both enzymes differed significantly. AmProt1 had a narrow pH optimum around 3, whereas AmProt2 had a broad pH optimum between 3 and 5.5 and a higher affinity to the substrate methylcasein. One cDNA-clone (AmProt1*) that presumably encodes AmProt1 was identified. Like AmProt1, this cDNA was expressed in a pH-dependent manner. In addition, carbohydrate and to a lesser extent nitrogen depletion significantly enhanced AmProt1* expression. In fully developed Populus hyb./A. muscaria ectomycorrhizas the expression of AmProt1* was significantly higher in hyphae of the Hartig net compared with those of the fungal sheath. The role of AmProt1 and AmProt2 for fungal physiology and competitiveness is discussed.