Three proline rotamases involved in calcium homeostasis play differential roles in stress tolerance, virulence and calcineurin regulation of Beauveria bassiana.

FK506-sensitive proline rotamases (FPRs), also known as FK506-binding proteins (FKBPs), can mediate immunosuppressive drug resistance in budding yeast but their physiological roles in filamentous fungi remain opaque. Here, we report that three FPRs (cytosolic/nuclear 12.15-kD Fpr1, membrane-associated 14.78-kD Fpr2 and nuclear 50.43-kD Fpr3) are all equally essential for cellular Ca homeostasis and contribute significantly to calcineurin activity at different levels in the insect-pathogenic fungus Beauveria bassiana although the deletion of fpr1 alone conferred resistance to FK506. Radial growth, conidiation, conidial viability and virulence were less compromised in the absence of fpr1 or fpr2 than in the absence of fpr3, which abolished almost all growth on scant media and reduced growth moderately on rich media. The Δfpr3 mutant was more sensitive to Na , K , Mn , Ca , Cu , metal chelate, heat shock and UVB irradiation than was Δfpr2 while both mutants were equally sensitive to Zn , Mg , Fe , H O and cell wall-perturbing agents. In contrast, the Δfpr1 mutant was less sensitive to fewer stress cues. Most of 32 examined genes involved in DNA damage repair, Na /K detoxification or osmotolerance and Ca homeostasis were downregulated sharply in Δfpr2 and Δfpr3 but rarely so affected in Δfpr1, coinciding well with their phenotypic changes. These findings uncover important, but differential, roles of three FPRs in the fungal adaptation to insect host and environment and provide novel insight into their essential roles in calcium signalling pathway.