Adhesion Plaques: Sites of Transmembrane Interaction Between the Extracellular Matrix and the Actin Cytoskeleton

Abstract

SUMMARYIn this paper we review what is known about the organization of adhesion plaques, the regions where cells in culture adhere most tightly to the underlying substratum. These specialized areas of the plasma membrane serve as attachment sites for stress fibres. A major objective has been to determine how microfilament bundles are anchored at such regions. In their morphology and composition adhesion plaques resemble the adhesions fibroblasts make to the extracellular matrix. Some extracellular matrix components have been identified on the outside face of adhesion plaques. Within the plasma membrane of adhesion plaques, extracellular matrix receptors, such as the fibronectin receptor (integrin), have been identified. This transmembrane glycoprotein complex has been shown to bind the cytoplasmic protein talin, which, in turn, associates with vinculin. These proteins establish a transmembrane chain of attachment between the extracellular matrix and the cytoskeleton, although how the actin filaments interact with these components remains to be determined. Besides having a structural function, adhesion plaques may also be regions where regulatory signals are transmitted across the membrane. Consistent with this idea has been the finding that various tyrosine kinases and a calcium-dependent protease are concentrated at the cytoplasmic aspect of adhesion plaques. Furthermore, several adhesion plaque proteins become phosphorylated during cell transformation by Rous sarcoma virus. In future work it will be important to determine how such modifications affect the interactions of these proteins and the stability of adhesion plaques.

Focal Adhesions

“…40 This point is underscored by several proteins classically thought to be "structural" FA components: vinculin is selectively activated by changes in head-tail interactions regulated by binding to talin 41,42 ; ␣-actinin forms a signaling complex with the Abl/Arg kinase adapter ArgBP2 43 ; and paxillin integrates diverse inputs including tyrosine kinases and Rho family regulators. 44 Additionally, FAs contain a rich diversity of enzymatically active proteins that direct cell fate, shape, and motion (see Figure 2).…”

Cell biology of arterial proteoglycans.

“…104 - 105 Most cells, including vascular endothelial cells, interact with their substrata in specialized areas of the cell known as focal adhesion sites. 116 - 119 These sites are enriched on the ECM side in attachment proteins, such as fibronectin and heparan sulfate proteoglycans. and on the cytoplasmic side in actin microfilaments.…”

“…and on the cytoplasmic side in actin microfilaments. 119 Colocalization of HSPG with cytoskeletal elements such as actin suggests that proteoglycans may influence cell adhesion through association with the cytoskeleton. 111116 Recent studies have shown that the integrity of actin stress filaments is critical for endothelial cell adherence when exposed to shear stress.…”

Biochemical localization of the transformation-sensitive 52 kDa (p52) protein to the substratum contact regions of cultured rat fibroblasts. Butyrate induction, characterization, and quantification of p52 in v-ras transformed cells

“…6). Structurally, SAM consists primarily of tight focal contacts and a subset of a close contacts (for review see Lark et al, 1985), which are distinguishable both in proximity to the substratum surface and in protein composition (for detailed structural analysis of these contact regions see Krueger et al, 1984;Ben-Ze'ev, 1985;Burridge et al, 1987). Cell-substrate contact sites were also prepared in the present study using the saponinextraction procedure.…”