The effects of pH on cortex and condensate formation.

The actin cortex enables cells to undergo shape changes in response to mechanical and chemical cues. Although F-actin and actin-associated proteins are present throughout cells, cortical structures assemble beneath the membrane, while actin-associated proteins form condensate-like structures in the cytoplasm. The mechanism regulating the separation between cortical and cytoplasmic phases remains unclear. Since many actin-associated proteins display pH-dependent interactions, intracellular pH gradients may contribute to this phase separation. Here, we investigate how pH regulates cortex assembly and condensate formation in Dictyostelium discoideum. We find that altering extracellular pH affects actin-associated protein's cortical localization and impacts their association with the actin network. Reconstitution assays indicate that cortexillin I-mediated actin crosslinking and bundling is enhanced at lower pH but can be affected by discoidin I, a weak, pH-dependent actin crosslinker. Finally, we found that the cell's cortical region is slightly more acidic than the cytoplasm, supporting a model in which intracellular pH helps regulate the balance between condensate assembly and cortex formation.