Transcription factors SltA and CrzA reversely regulate calcium homeostasis under calcium-limited conditions

Abstract

ABSTRACT The maintenance of cellular calcium homeostasis in response to calcium supply is required for a wide range of physiological processes in fungi, including growth, development, and virulence. In Aspergillus , the transcription factor CrzA is involved in efficient regulation of calcium homeostasis, especially under calcium-repleted conditions. However, the transcriptional regulatory mechanisms under calcium-limited conditions remain unclear. Here, we found that a highly conserved transcription factor, SltA, known for its roles in salt tolerance and azole resistance, can confer adaptation to calcium-limited conditions in Aspergillus fumigatus . Loss of sltA caused severe growth defects with abnormal expressions of calcium metabolism-related genes, which resulted in a decreased cytosolic calcium transient under calcium-limited conditions, while the addition of calcium rescued all defective phenotypes in the Δ sltA mutant. Moreover, A. fumigatus SltA undergoes a proteolysis modification that depends on SltB, a chymotrypsin-like serine protease. Strikingly, A. fumigatus SltA plays a reverse role to CrzA in the regulation of expressions of calcium metabolism-related genes. Deletion of crzA significantly suppressed the growth defect of the Δ sltA mutant. Deletion of sltA led to highly expressed CrzA with a constant nuclear location under calcium-limited conditions, demonstrating that there exists a mutually restricted network regulated by SltA and CrzA to adapt to calcium-limited conditions. Electrophoretic mobility shift assays and chromatin immunoprecipitation sequencing revealed that SltA directly binds to a subset of genes involved in calcium metabolism via a conserved motif. These findings expand knowledge on the regulatory circuits of the Ca 2+ signaling pathway. IMPORTANCE Calcium ions are ubiquitous intracellular signaling molecules for many signaling pathways regulating the fungal response to stress and antifungal drugs. The concentration of intracellular calcium is tightly regulated in its storage, release, and distribution. CrzA is the best-studied transcription factor that regulates this process under sufficient calcium or other external signals. However, CrzA was excluded from nuclei and then lost transcriptional activation under calcium-limited conditions. The regulators in the Ca 2+ signaling pathway under calcium-limited conditions remain unclear. Here, we identified SltA as a key regulator in the Ca 2+ signaling pathway under calcium-limited conditions, and the underlying mechanisms were further explored in Aspergillus fumigatus . These findings reveal a transcriptional control pathway that precisely regulates calcium homeostasis under calcium-limited conditions.