One way to analyze a complex multifunctional protein molecule such as fibronectin is to synthesize individual regions of the protein and test their ability to bind other proteins. By these and other methods, it was possible to show that one region of fibronectin binds to collagen, another to proteoglycans, and another to specific integrins on the surface of various types of cells (see Figure 19–47B). Synthetic peptides corresponding to different segments of the integrin-binding domain were then used to show that binding depends on a specific tripeptide sequence (Arg-Gly-Asp, or RGD) that is found in one of the type III repeats (see Figure 19–47C). Even very short peptides containing this RGD sequence can compete with fibronectin for the binding site on cells, thereby inhibiting the attachment of the cells to a fibronectin matrix.
Several extracellular proteins besides fibronectin also have an RGD sequence that mediates cell-surface binding. Many of these proteins are components of the extracellular matrix, while others are involved in blood clotting. Peptides
Figure 19–47 The structure of a fibronectin dimer. (a) Electron micrographs of individual fibronectin dimer molecules shadowed with platinum; red arrows mark the joined C-termini. (B) the two polypeptide chains are similar but generally not identical (being made from the same gene but from differently spliced mrNas). they are joined by two disulfide bonds near the C-termini. Each chain is almost 2500 amino acids long and is folded into multiple domains (see Figure 19–46). as indicated, some domains are specialized for binding to a particular molecule. For simplicity, not all of the known binding sites are shown. (C) the three-dimensional structure of the ninth and tenth type III fibronectin repeats, as determined by x-ray crystallography. Both the arg-Glyasp (rGD) and the “synergy” sequences shown in red are important for binding to integrins on cell surfaces. (a, from J. Engel et al., J. Mol. Biol. 150:97–120, 1981. With permission from academic press; C, from Daniel J. Leahy, Annu. Rev. Cell Dev. Biol. 13:363–393, 1997. With permission from annual reviews.) 1068 Chapter 19: Cell Junctions and the Extracellular Matrix containing the RGD sequence have been useful in the development of anti-clotting drugs. Some snakes use a similar strategy to cause their victims to bleed: they secrete RGD-containing anti-clotting proteins called disintegrins into their venom.
The cell-surface receptors that bind RGD-containing proteins are members of the integrin family, which we describe in detail later. Each integrin specifically recognizes its own small set of matrix molecules, indicating that tight binding requires more than just the RGD sequence. Moreover, RGD sequences are not the only sequence motifs used for binding to integrins: many integrins recognize and bind to other motifs instead.
tension Exerted by Cells regulates the assembly of Fibronectin Fibrils