Actin (blue) and microtubules (red) come together thanks to Arg (green).

A ctin both pushes out the membrane of moving cells and links to adhesion sites to give those cells traction. But actin does not operate alone. Microtubules (MTs) are targeted to the front of the cell to deliver signals that promote actin polymerization and thus give moving cells directionality. Now, Miller et al. (page 407) report that Arg, a nonreceptor tyrosine kinase, acts as a physical link between actin and MTs that may allow this signal transfer.

Arg was known to bind and bundle actin, but the report of a MT-binding site between the two actin-binding sites is new. MTs cosediment with but are not cross-linked by Arg; cross-links do, however, form between arrays of MTs and F-actin when Arg is present. Arg staining coincides with concentrations of both F-actin and MTs at protrusive structures at the cell periphery.

Cells with no Arg or only a version of Arg that cannot bind MTs show very few lamellipodial protrusions, with fewer MTs targeting any actin clusters. Arg may guide growing MTs along actin bundles or help secure MTs once they have arrived at an adhesive structure. The MTs would then deliver their signal, and Arg could target its recently identified phosphorylation substrate p190RhoGAP, resulting in less Rho activity and thus more protrusion. ▪