The Cdc42 Effector Kinase PAK4 Localizes to Cell-Cell Junctions and Contributes to Establishing Cell Polarity

Area of Science:

• Cell Biology
• Molecular Biology
• Cancer Research

Background:

• The serine/threonine kinase PAK4 is a Cdc42 effector with an unclear role, though its overexpression is linked to cancers and increased cell migration.
• Previous studies suggest a correlation between PAK4 levels and enhanced cell migration in vitro.

Purpose of the Study:

• To investigate the precise role of PAK4 in cell migration, polarization, and its association with cell-cell junctions.
• To identify downstream targets of PAK4 and its function in the context of Cdc42 signaling.

Main Methods:

• Utilized cell lines (U2OS, MCF-7) for PAK4 and Cdc42 depletion experiments.
• Employed PAK4 inhibitor PF-3758309 to assess effects on cell migration, polarization, and centrosome reorientation.
• Confirmed beta-catenin as a PAK4 target by examining its phosphorylation status.

Main Results:

• PAK4 predominantly localizes to cell-cell junctions, not focal adhesions or leading edges.
• PAK4 depletion did not impair collective cell migration but affected cell polarization.
• PAK4 depletion or inhibition led to defects in centrosome reorientation and inhibited beta-catenin Ser-675 phosphorylation at cell-cell junctions.

Conclusions:

• PAK4 is a component of a conserved cell-cell junctional polarity Cdc42 complex, regulating cell polarization.
• PAK4's primary role appears to be in cell polarity rather than direct promotion of cell migration.
• PAK4 directly targets beta-catenin for phosphorylation at cell-cell junctions, impacting polarity mechanisms.