Abstract
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, Ca(2+)-dependent phospholipase from the mycorrhizal fungus Tuber borchii. TbSP1 is the first phospholipase A(2) 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 A(2) 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 A(2) in the organization of multicellular filamentous structures in bacteria and fungi.