Lim et al. now clear up the picture.
As a cell crawls along a surface, it temporarily attaches at sites called focal adhesions. FAK concentrates at focal adhesions and passes on signals from integrins that have latched onto molecules in the extracellular matrix. Fibroblasts missing FAK remain rounded up instead of flattening out on a surface and crawl sluggishly. Both of which might be expected if focal adhesions are impaired. However, FAK-lacking cells actually have an inordinate number of focal adhesions and have increased RhoA activity—a factor that spurs focal adhesion and stress fiber formation, thus promoting the rounded shape by increasing the internal contractile force.
Lim et al. now show that some of these observations can be explained by the fact that when FAK is lacking, fibroblasts manufacture more of FAK's paralogue, proline-rich kinase (Pyk2).
To sort out Pyk2's contributions, Lim et al. used RNAi to knock down the protein in fibroblasts that also lacked FAK. This reduction restored normal focal adhesions and RhoA activity pattern and reverted cells to a flattened shape.
The FAK- and Pyk2-lacking cells continued to have a motility problem, however. The cells couldn't detach their rear ends from the surface, the team found. The cells stretched and stretched until their anchored back ends broke off and the rest of their cell bodies snapped forward like rubber bands. That result indicates that the immobility of FAK-lacking cells isn't due to extra Pyk2.
Exactly why Pyk2 gets up-regulated in FAK-deficient cells, and how overexpression of PyK2 causes constitutive RhoA expression, is unclear. In any case, the work suggests that researchers should take a second look at conclusions based on FAK-deficient cells.