Aspergillus nidulans Ambient pH Signaling Does Not Require Endocytosis

Area of Science:

• Molecular biology
• Cell biology
• Mycology

Background:

• The Aspergillus nidulans alkaline pH signaling (Pal) pathway regulates gene expression in response to external pH.
• This pathway involves the ESCRT machinery and the arrestin-like protein PalF, leading to PacC transcription factor processing.
• The localization and activation mechanism of the PalB protease, crucial for the initial pH-regulated cleavage of PacC, remained unclear, with endocytosis as a potential requirement.

Purpose of the Study:

• To investigate the role of endocytosis in the activation of the Aspergillus nidulans Pal pathway.
• To determine the localization of the PalB protease in response to alkaline pH.
• To clarify whether ESCRT-mediated signaling in the Pal pathway involves endocytic internalization.

Main Methods:

• Utilized a SynA internalization assay to assess the impact of various endocytic mutations on endocytosis.
• Tested the functional activation of the Pal pathway in fungal strains with impaired endocytosis.
• Determined the subcellular localization of the PalB protease under alkaline pH conditions using microscopy.

Main Results:

• Mutations affecting endocytosis, including a conditional complete block mutant (slaB1), did not impede the activation of the Pal pathway.
• The PalB protease, similar to PalA and PalC, was found to localize to cortical structures in a pH-dependent manner.
• These findings indicate that endocytic internalization is not required for Pal pathway activation.

Conclusions:

• The Pal pathway in Aspergillus nidulans operates independently of endocytosis.
• PalB protease localization and activation occur at cortical structures, likely associated with the plasma membrane.
• This study refutes the hypothesis that endocytic internalization is necessary for alkaline pH signaling in fungi.