Self-processing of the Phospholipase A2 from the Symbiotic Ascomycete Tuber borchii

TbSP1 is a secreted phospholipase A2 expressed in the filamentous fungus Tuber borchii and is up-regulated by nutrient starvation. In this study, the occurrence of an autoproteolytic processing, both in vivo and in vitro, is reported for TbSP1. The cleavage takes place at the N-terminus of the protein via an intermolecular mechanism, and generates a fragment with molecular mass of 14.5kDa. Distinct catalytic residues are involved in the two enzymatic activities, since point mutant in the His responsible for phospholipid hydrolysis still retains self-processing ability. TbSP1 autoproteolysis is presumably mediated by a Ser residue, however the precise localization and the structure of the active site are still unknown. The combination of limited proteolysis assays, site-directed mutagenesis and the synthesis of a chimeric Trx construct suggested that both a structural and a sequence motif play a role in the recognition of the cleavage target region. Since the digestion fragment has increased phospholipasic and proteolytic activity, a physiological role has been proposed for the auto-proteolytic event, which thus represents a further step in TbSP1 maturation process.