Focal adhesion proteins associated with apical stress fibers of human fibroblasts

AbstractHuman fibroblasts stained with fluorescently labeled phalloidin revealed many stress fibers within the apical cytoplasm in addition to those located along the basal plasma membrane and associated with focal adhesions. The staining patterns of these apical stress fibers with fluorescent phalloidin, anti‐α‐actinin, and anti‐myosin were identical to those of the basal stress fibers, suggesting the same macromolecular organization for both types f stress fibers. There were two types of apical stress fibers that clearly interacted with the apical plasma membrane, those extending between the basal and the apical plasma membrane and those having both ends on the basal membrane forming arches whose top interacted with the apical plasma membrane. By electron microscopy, we observed that apical stress fibers were associated with the apical plasma membrane via electron‐dense plaques reminiscent of the focal adhesion. Since several proteins have been specifically localized to the focal adhesion site, we examined whether they were also present at the apical stress fiber‐membrane association site by using immunocy‐tochemical methods and image reconstruction techniques. We found that vinculin, talin, paxillin, a fibronectin receptor protein, several integrin subunits including β1, fibronectin, and proteins with phosphorylated tyrosine were also components of the apical plaque. These observations indicate that apical stress fibers are attached to the plasma membrane by using principally the same molecular assembly as the focal adhesion associated with the basal stress fiber. We suggest that the complex molecular organization of the focal adhesion is not demanded by cell adhesion, but rather it is needed for anchoring stress fibers to the plasma membrane. Apical plaques did not stain with the anti‐integrin αv subunit or anti‐focal adhesion associated kinase (FAK), although these antibodies stained focal adhesions. These results suggest that the apical stress fiber‐membrane contact has some important functions different from those of the focal adhesion.