Fibronectin-binding 36 kDa protein in human fibroblasts

Extracellular glycoproteins at acetylcholine receptor clusters of rat myotubes are organized into domains

Cultured neonatal rat myotubes develop acetylcholine receptor (AChR) clusters where they adhere to the substrate. These clusters are often linear, with AChR-rich domains alternating with AChR-poor "contact domains" closer to the tissue culture substrate. We have used sequential detergent extraction and immunofluorescence microscopy to localize extracellular matrix components within these two domains. Laminin, heparan sulfate proteoglycan, type IV collagen, and fibronectin are present at AChR-rich domains; nerve cell adhesion molecule is present at both AChR and contact domains. Extracts of contact domains are enriched in a 36-kDa concanavalin A binding protein and in a 90-kDa polypeptide recognized by antibodies against rat muscle adherons. These results suggest that extracellular components at substrate-apposed AChR clusters are organized into distinct domains that parallel the organization of the cluster bilayer.

The involvement of the cell matrix receptors, or VLA integrins, in the morphogenetic behavior of normal and malignant cells is gradually being uncovered

The VLA-Integrins are members of a family of cell surface receptors that recognize Arg-Gly-Asp containing ligands and that allow cells to bind extracellular matrix molecules such as fibronectin, laminin, and collagens. These interactions reorganize the topographic distribution of the receptors at the cell surface and are relayed to the cytoplasm and even to the cell nucleus. Coordinated organization of the intracellular cytoskeleton, cell spreading on the matrix, and movement of the cells along concentration gradients of these matrix molecules are the result of these interactions. Evidence exists that the migration of embryonic cells, platelet activation on endothelial defects, and the invasion of fibroblasts, lymphoid cells, and epithelial cells during wound healing are facilitated by Integrin-matrix interactions. Moreover, deficiency in the synthesis of a particular Integrin subunit or the inappropriate expression of one or a few members of this family might contribute fundamentally to invasion and metastasis of malignant cells. Although extensive information is available on the structure of the common beta 1 subunit and on the function of the fibronectin receptor, in particular, the other five members of the very late antigen family, their respective alpha subunits and their function as cell surface receptors remain poorly understood. The rationale for having at least six different receptors on some cells, how they might collaborate, and to what end remains unknown. Finally, the interaction of these cell surface receptors and the modulation of their affinity for their ligand by other cell surface or extracellular matrix components such as heparan sulfate, gangliosides a.o. have been barely touched upon. It can be predicted that these molecules and their interaction with the extracellular matrix will provide a key to the understanding of many normal and malignant morphogenetic cellular properties.