Focal contacts

Stress fibers are parallel bundles of actin filaments that develop in the cytoplasm of fibroblasts from the cortical actin network in response to mechanical tension. These often bind to the plasma membrane at focal contacts and, through transmembrane linker glycoproteins, to the extracellular matrix. Thus, actin filaments of stress fibers indirectly Join to the inner face of the plasma membrane through molecular assemblies of attachment proteins, which include an actin-capping protein, a-actinin, vinculin, and talin (Small, 1988). 

The tight junction is a component of the junctional complexes which join cells. Immediately basolateral to the tight junction is the zonula adherens (Figs. 6 and 7). Because the zonula adherens and the gap junctions are focal contact regions, they do not impact transport by the paracellular pathway. All of these junctions are specialized regions of the lateral cell membrane which demarcate the lateral space. In certain types of cells the lateral space is rather narrow and... 

Duband, J.-L., Nuckolls, G., Ishihara, A., Hasegawa, T., Yamada, K., Thiery, J.P., and Jacobson, K. (1988) Fibronectin receptor exhibits high lateral mobility in embryonic locomoting cells but is immobile in focal contacts and fibrillar streaks in stationary cells./. Cell Biol. 107, 1385-1396. 

Shekel, S.K., and Wang, Y.-L. (1988) Synthetic peptide GRGDS induces dissociation of alpha-acdn and vinculin from the sites of focal contacts./. Cell Biol. 107, 1231. 

VASP Inhibits VASP binding to F-actin inhibits VASP localization to focal contacts and integrins... 

Quantitative determination of the absolute distance from the surface to a labeled cell membrane at a cell/substrate contact region can be based on the variation of F(d) with 0.(1O6) This effort is challenging because corrections have to be made for 0-dependent reflection and transmission through four stratified layers (glass, culture medium, membrane, and cytoplasm), all with different refractive indices. For 3T3 cells, Lanni et a//1065 derived a plasma membrane/substrate spacing of 49 nm for focal contacts and 69 nm for close contacts elsewhere. They were also able to calculate an approximate refractive index for the cytoplasm of 1.358 to 1.374. 

Schematic illustration of a so-called focal contact, showing how extracellular fibronectin is believed to be indirectly attached to the intracellular cytoskeleton through a transmembrane fibronectin receptor and several other peripheral membrane proteins.

Plakins are not the only proteins that tether IFs to the cell surface. A number of other proteins localized to cell surface structures including desmosomes, focal contacts, and muscle costameres also contribute to IF anchorage at plasma membranes. IFs may also associate with cell surface receptors outside of ultrastructurally distinct structures, a topic that will be dealt with below. 

The cortical region of many cells is enriched in actin and associated actin-binding proteins, which function in motility, cell shape maintenance, and membrane protein distribution in polarized cells. In some cases, discrete structures anchor actin to the membrane, as is the case for intercellular adherens junctions and cell-substrate focal contacts. In certain special cell types, the fundamental blueprint for an adherens junction is taken to a new structural level, serving as scaffolding for cell-type specific complexes, such as the dystrophin-associated protein complex (DPC) in striated muscle. Although for years morphological studies have described a close association with IF with the actin-rich cortex, recent advances in methods to study protein-protein interactions have provided new insight into the intimate structural and functional relationship between IFs and these membrane domains. 

Fibroblasts and endothelial cells do not assemble hemidesmosomes. Rather, their matrix adhesive devices are so-called focal contacts. Pivotal components of focal contacts are heterodimeric integrins that bind to a... 

Seifert, G. J., Lawson, D., and Wiche, G. (1992). Immunolocalization of the intermediate filament-associated protein plectin at focal contacts and actin stress fibers. Eur. J. Cell Biol. 59, 138-147. 

Tsuruta, D., and Jones, J. G. (2003). The vimentin cytoskeleton regulates focal contact size and adhesion of endothelial cells subjected to shear stress. J. Cell Sci. 116, 4977-4984. 

Fig. 8 (a) Nanopatteming by EBL A silcion substrate is functionalized with an amino-silane and coated with BSA. A focused electron beam is employed to write nanopattems into the BSA film. Proteins from solution can selectively adsorb into the nanopattems and guide the formation of cell-substrate contacts, (b) Fibroblast on fibronectin 10 x 10 nanodot matrix created by electron beam lithography. Cells spread, and fluorescent staining of intracellular proteins shows that focal contacts are located on the nanodots actin green), fibronectin red), and vinculin blue). Areas a, b, and c are shown magnified. (Figure in part reproduced with permission from [103])... 

Geiger B, Bershadsky A (2001) Assembly and mechanosensory function of focal contacts. Curr Opin Cell Biol 13 584-592... 

Massia SP, Hubbell JA (1991) An RGD spacing of 440nm is sufficient for integrin alpha-V-beta-3-mediated fibroblast spreading and 140 nm for focal contact and stress fiber formation. J Cell Biol 114(5) 1089-1100... 

The manner by which shear stress-induced cellular changes occur in endothelial cells involves cell membrane and cytoskeletal molecules that lead to a shape change. The cytoskeleton contains actin filaments, intermediate filaments, and microtubules, all of which are restructured upon exposure to external force. Under stress conditions, actin filaments coalesce to form stress fibers that anchor at the focal contacts, which are adhesion sites at the cell substrate interface. 

Guignandon A, Usson Y, Laroche N, Lafrage-Proust MH, Sabido O, Alexandre C, Vico L. Effects of intermittent or continuous gravitational stresses on cell-matrix adhesion Quantitative analysis of focal contacts in osteoblastic ROS 17/2.8 cells. Exp Cell Res. 1997 10 66-75. 

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