SakA and MpkC interact with PkaAC1 to regulate the mobilization of carbon sources required for cell wall biosynthesis in Aspergillus fumigatus
In Aspergillus fumigatus, sensing of cell wall-related stresses and subsequent signaling pathways involve mitogen-activated protein kinases (MAPKs). In A. fumigatus there are four MAPKs: MpkA, MpkB, MpkC and SakA. These kinases are involved in maintaining the normal morphology of the cell wall and in the resistance against cell wall-damaging agents. The main components of A. fumigatus cell wall are chitin, and glucans. Upon cell wall stress, cell wall-related sugars are synthesized from intracellular carbohydrate storage compounds. Carbohydrate sensing and stress response are regulated by protein kinase A (PKA). In A. fumigatus, PKA contains two catalytic subunits named PkaAC1 and PkaAC2; PkaC1 seems to be the main catalytic subunit. This work shows that glycogen, trehalose content and PKA activity were reduced in the sakA and sakA/mpkC null deletion mutants. Overexpression of pkaAC1 shows reduced levels of trehalose and glycogen and the treatment with Congo red induces glycogen degradation but not trehalose, showing that glycogen degradation is required to the response to Congo red. The mobilization of trehalose and glycogen go through phosphoglucomutase (PgmA) and hexo/glucokinases. The sakA and sakA/mpkC null mutants showed reduced PgmA activity while a hxkA/glkA mutant is very sensitive to cell wall-damaging agents, osmotic stress and it has attenuated virulence in neutropenic and non-neutropenic mouse models. Finally, the uptake of external glucose was reduced in sakA and sakA/mpkC null mutants and N-acetylglucosamine and glucose concentration was reduced in their cell walls. These results suggest that the reduced mobilization of simple sugars impairs the structure of the cell wall. In summary, we propose that SakA and MpkC are important for the modulation of PKA activity, therefore regulating the availability and mobilization of monosaccharides for cell wall biosynthesis during cell wall damage response.
Keywords: SakA, MpkC, pkaAC1, PKA, cell wall, HogA, glycogen, trehalose.
Financial support: FAPESP and CNPq, Brazil