# Human Plastins are Novel Cytoskeletal pH Sensors with a Reduced F-actin Bundling Capacity at Basic pH

## TL;DR

This study shows that human plastins respond to changes in cell pH by altering their ability to bundle actin, which may influence cancer cell behavior.

## Contribution

Human plastins are identified as novel cytoskeletal pH sensors with reduced F-actin bundling at basic pH.

## Key findings

- PLS2 and PLS3 exhibit reduced F-actin bundling at alkaline pH due to decreased ABD1 affinity.
- Elevated pH causes PLS2 and PLS3 to dissociate from actin structures in fibroblast cells.
- His207 is a key pH-sensing residue in PLS2 that affects pH sensitivity when mutated.

## Abstract

Intracellular pH (pHi) is a fundamental component of cell homeostasis. Controlled elevations in pHi precede and accompany cell polarization, cytokinesis, and directional migration. pH dysregulation contributes to cancer, neurodegenerative diseases, diabetes, and other metabolic disorders. While cytoskeletal rearrangements are crucial for these processes, only a few cytoskeletal proteins, namely CDC42, cofilin, talin, cortactin, α-actinin, and AIP1 have been documented as pH sensors. Here, we report that actin-bundling proteins plastin 2 (PLS2, aka LCP1) and plastin 3 (PLS3) respond to physiological scale pH fluctuations by a reduced F-actin bundling at alkaline pH. The inhibition of PLS2 actin-bundling activity at elevated pH stems from the reduced affinity of the N-terminal actin-binding domain (ABD1) to actin. In fibroblast cells, elevated cytosolic pH caused the dissociation of ectopically expressed PLS2 and 3 from actin structures, whereas acidic conditions promoted their tighter association with focal adhesions and stress fibers. We identified His207 as one of the pH-sensing residues of PLS2 whose mutation to Lys and Tyr reduces pH sensitivity by enhancing and inhibiting the bundling ability, respectively. Our results suggest that weaker actin bundling by plastin isoforms at alkaline pH favors higher dynamics of the actin cytoskeleton. Therefore, like other cytoskeleton pH sensors, plastins promote disassembly and faster dynamics of cytoskeletal components during cytokinesis and cell migration. Since both plastins are implemented in cancer, their pH sensitivity may contribute to the accelerated proliferation and enhanced invasive and metastatic potentials of cancer cells at alkaline pHi.