These sideways movements were picked up by the authors as ripples in the plasma membrane at the very front of a migrating fibroblast. The ripples were made by polymerizing actin filaments that elongated nearly parallel to the lamellipodial leading edge. Their quick polymerization moved the ripples several times faster than lamellipodia can extend forward.
The tips of the actin filaments at the ripple crests were attached to ready-to-bind adhesion receptors. Although not yet bound to the matrix, these β1 integrins were in a conformation that readily binds to fibronectin. Actin polymerization in the unique environment of the extreme leading edge, where adhesions are lacking, seems somehow to drive their active conformation; decreasing polymerization reduced the number of activated receptors.
Similar activated integrins were found by the group at the tips of long searching filopodia at the front of neuronal growth cones. “It's easiest to picture sticky fingers in the growth cones,” says Catherine Galbraith. “They're how cells feel along, looking for good adhesion sites.” These sites have the proper stiffness and the right matrix ligands to initiate formation of adhesion foci.