From Nat. Cell Biol. 11, 624–630 (2009).

Talin is essential for integrin activation and focal adhesion formation. In migrating cells, it is cleaved by calpain into a rod domain and a head domain that binds -integrin tails. The head domain contains a potential phosphorylation site for Cdk5 — a kinase known to influence cell migration. Reporting in Nature Cell Biology, Mark Ginsberg, Ken Jacobson, and colleagues now show that Cdk5-dependent phosphorylation of talin's head domain regulates its ubiquitylation and degradation via the E3 ligase Smurf1, thus controlling adhesion turnover and migration.

The authors first showed that Cdk5 phosphorylates the talin head domain at Ser 425 both in vitro and in vivo. Talin is known to regulate focal adhesion dynamics, so they next analysed the turnover of two talin mutants fused to enhanced green fluorescent protein (EGFP) — the non-phosphorylatable tal^S425A and the phosphomimetic tal^S425D — at protrusions of Chinese hamster ovary (CHO) cells. Focal adhesion disassembly was increased in the presence of EGFP-tal^S425A and decreased in the presence of EGFP- tal^S425D. Furthermore, the reduction in talin phosphorylation prevented protrusion stabilization, whereas EGFP- tal^S425D expression resulted in increased protrusion persistence and excessively firm adhesion to the substrate, leading to impaired cell migration in both cases. This effect was not cell-type-specific as phosphorylation at talin Ser 425 was also confirmed to be important for the migration of neuronal cells.

But how does Cdk5-mediated talin phosphorylation regulate focal adhesion disassembly? The authors focused on the role of Smurf1 — an E3 ubiquitin ligase that targets proteins for proteasomal degradation and localizes to focal adhesions. Smurf1 bound tightly to the talin head domain but not full-length talin both in vitro and in vivo, and this interaction was affected by Cdk5 activity. In vitro phosphorylation inhibited the binding of talin head domain to Smurf1, whereas mutated Ser 425 increased binding. This suggests that phosphorylation of talin by Cdk5 inhibits its interaction with Smurf1. Levels of the talin head domain also decreased when co-transfected with Smurf1 into CHO cells, but degradation was inhibited upon treatment with a proteasome inhibitor, showing that Smurf1–mediated ubiquitylation leads to degradation of the head domain. The authors confirmed that ubiquitylation of talin is mediated by Smurf1 and inhibited upon phosphorylation by Cdk5 in vivo.

This work shows that Cdk5-mediated phosphorylation of the talin head domain prevents its interaction with Smurf1 and degradation. The authors also show that the subsequent degradation of the talin head domain promotes focal adhesion turnover. Thus, regulation of talin by Cdk5 stabilizes adhesions and lamellipodia at the front of cells, leading to optimal rates of cell migration.

Author: Kim Baumann