Chinthalapudi et al. reveal how the phospholipid PIP2 induces oligomerization of the focal adhesion protein vinculin to promote adhesion turnover and cell migration.
Vinculin stabilizes nascent focal adhesions between the cell and ECM by linking them to the actin cytoskeleton. Vinculin also binds to phosphatidylinositol 4,5-bisphosphate (PIP2), which is enriched at focal adhesions, but how this phospholipid regulates vinculin’s function is unclear.
Chinthalapudi et al. obtained a crystal structure of vinculin’s C-terminal tail bound to PIP2 and found that the phospholipid was sandwiched between three vinculin molecules, inducing a subtle conformational change that promoted vinculin oligomerization. A previous study suggested that PIP2 prevents vinculin from binding to F-actin, but Chinthalapudi et al.’s structure revealed that vinculin’s PIP2- and actin-binding sites are distinct, suggesting that vinculin may be able to bind to both molecules simultaneously.
The researchers identified single point mutations that inhibited vinculin’s ability to bind PIP2 without affecting the protein’s interaction with actin. Although these PIP2 binding-deficient mutants localized to focal adhesions, they were unable to rescue the disorganized actin filaments formed in vinculin-deficient fibroblasts. Moreover, whereas vinculin-null cells migrate faster than wild-type cells, fibroblasts expressing PIP2-binding mutants moved very slowly, suggesting that their adhesions might be hyperstabilized. Indeed, photobleaching experiments demonstrated that vinculin mutants unable to bind PIP2 were immobilized at focal adhesions, indicating that PIP2 is required for vinculin turnover. Senior author Tina Izard now wants to investigate whether PIP2 also promotes the oligomerization and function of other actin-binding proteins.
Text by Ben Short