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A nutrient‐regulated, dual localization phospholipase A2 in the symbiotic fungus Tuber borchii

Important morphogenetic transitions in fungi are triggered by starvation‐induced changes in the expression of structural surface proteins. Here, we report that nutrient deprivation causes a strong and reversible up‐regulation of TbSP1, a surface‐associated, Ca2+‐dependent phospholipase from the mycorrhizal fungus Tuber borchii. TbSP1 is the first phospholipase A2 to be described in fungi and identifies a novel class of phospholipid‐hydrolyzing enzymes. The TbSP1 phospholipase, which is synthesized initially as a pre‐protein, is processed efficiently and secreted during the mycelial phase. The mature protein, however, also localizes to the inner cell wall layer, close to the plasma membrane, in both free‐living and symbiosis‐engaged hyphae. It thus appears that a dual localization phospholipase A2 is involved in the adaptation of a symbiotic fungus to conditions of persistent nutritional limitation. Moreover, the fact that TbSP1‐related sequences are present in Streptomyces and Neurospora, and not in wholly sequenced non‐filamentous microorganisms, points to a general role for TbSP1 phospholipases A2 in the organization of multicellular filamentous structures in bacteria and fungi.