Differences in the stress fibers between fibrblasts and epithelial cells

Abstract

In the stress fibers of two types of nonmuscle cells, epithelia (PtK2, bovine lens) and fibroblasts (Gerbil fibroma, WI-38, primary human) the spacing between sites of α-actinin localization differs by a factor of about 1.6 as determined by indirect immunofluorescence and ultrastructural localization with peroxidase-labeled antibody. Both methods reveal striations along the stress fibers with a center-to-center spacing in the range of 0.9 μm in epithelial cells and 1.5 μm in fibroblasts. Periodic densities spaced at comparable distances are seen in PtK2 and in gerbil fibroma cells when they are treated with tannic acid and examined in the electron microscope. In such cells, densities are found not only along stress fibers but also at cell-cell junctions, attachment plaques, and foci from which stress fibers radiate. These latter three sites all stain with α-actinin antibody on the light and electron microscopy level. Stress fibers in the two cell types also vary in the periodicity produced by indirect immunofluorescence with tropomyosin antibodies. As is the case for α-actinin, the tropomyosin center-to-center banding is approximately 1.6 times as long in gerbil fibroma cells (1.7 μm) as it is in PtK2 cells (1.0 μm). These results suggest that the densities seen in the electron microscope are sites of α-actinin localization and that the proteins in stress fibers have an arrangement similar to that in striated muscle. We propose a sarcomeric model of stress fiber structure based on light and electron microscopic findings.