Mechanistic insights into the role of Ca ²⁺ -stimulated AMPK in the secretion of cellulases during carbon stress

The response of filamentous fungi towards recalcitrant carbohydrates is majorly governed by transcriptional activators of cellulase genes; however, little is known about the downstream events beyond transcription. We show here in Penicillium funiculosum that increasing the expression of a transcriptional activator CLR-2 in the catabolically derepressed strain, ΔMig1, didn’t exhibit a synergistic effect on cellulase production unless Ca ²⁺ was simultaneously increased. The RNA-seq screen for Ca ²⁺ -activated kinases identified SNF1-AMPK and SSP1-AMPKK as being specific to cellulose induction. Deletion of snf1 led to negligible secretion of cellulase upon induction. Quantitative whole-cell proteomics followed by chemical-genetic experiments with snf1- deleted strain showed that Ca ²⁺ -signaling channelizes carbon, nitrogen and energy sources towards cellulase production. Further, Ca ²⁺ -signaling phosphorylates SNF1-AMPK via SSP1, which in turn downregulates the phospho-HOG1 levels, leading to stimulus for cellulase secretion during carbon stress. The findings reported here are significant for understanding fungal pathology and developing second-generation biorefineries.