Activated Rho recruits p120ctn to cell–cell adhesions.

Calautti et al. have taken a collection of proteins that affect cell adhesion in keratinocytes and linked them into a pathway (page 137). The result is a simple sequence from Rho through to the phosphorylation of catenins associated with adherens junctions. But there are hints that reality may be more complex, with more side-branches and collaborations.

During differentiation of keratinocytes these skin cells strengthen their attachment to one another, and both Fyn kinase and Rho are activated. The ultimate effect of activating Rho—phosphorylation of catenins—requires Fyn. Calautti et al. link Rho and Fyn into the same pathway by showing that both activation of Rho and the interaction of Rho with the PRK2 kinase are sufficient and, in the case of Rho, required to turn on Fyn. PRK2, a Rho effector, increases Fyn activity after being added to a Fyn immunoprecipitate, although this effect may be indirect. Thus the pathway leads through Rho, PRK2, and Fyn, leading, finally, to catenin tyrosine phosphorylation and establishment of cell–cell adhesion.

The same research group has previously shown that a PRK2 relative called PKC-η is activated during keratinocyte differentiation and can phosphorylate Fyn. But this does not appear to result in catenin phosphorylation. Calautti et al. suggest that PRK2–Fyn may take care of catenins, while in another part of the cell PKC-η–Fyn may induce other aspects of terminal differentiation, such as a halt in cell division. In fact, two pools of Fyn exist in the keratinocytes, one of which is found in association with E-cadherin at cell–cell borders, and the other in the cytoplasm. Additional complications may arise in the PRK2–Fyn pathway, where other Rho-effector proteins may be needed to reinforce and sustain the signal responsible for Fyn activation. ▪