The fibronexus: a transmembrane association of fibronectin-containing fibers and bundles of 5 nm microfilaments in hamster and human fibroblasts.

A possible connection between external fibronectin-containing fibers and cytoplasmic 5 nm actin microfilaments within dense submembranous plaques has been observed by transmission electron microscopy. We refer to this transmembranous association as the fibronexus. Hamster embryo fibroblasts, transformed by wild-type or temperature-sensitive mutant (A28) SV40 virus, and human lung fibroblasts (WI-38, MRC-5) were studied using the tannic acid method of Simionescu and Simionescu (1976), which preferentially stained external carbohydrates. Fibronectin antigens were also localized on the extracellular fibers of the fibronexus with fibronectin antibody and immunoferritin staining. Goniometric studies of sections cut parallel to the plasmalemma demonstrated that the actin- and fibronectin-containing fibers of the fibronexus remained colinear when the specimen was tilted through a 40 degree angle about the fibrillar long axis. Sections cut perpendicular to the cell surface also showed that these fibers were apparently colinear. Our results suggest that the fibronectin and actin fibers of the fibronexus are closely associated (maximum separation distances of 8--22 nm), if not co-axial. Fibronexuses remained after expression of SV40-induced transformation, despite alteration of microfilament bundles and reduction in the amount of fibronectin (observed by immunofluorescence microscopy). The possible roles of fibronectin and the fibronexus in regulating actin polymerization are discussed.