Chapter 15, Problem 15.2PS

Problem Set

Anchoring Cells to the ECM. Animal cells attach to several different kinds of proteins within the ECM.

(a)    Briefly explain how the various domains of the fibronectin molecule (see Figure 15-16) or the laminin molecule (see Figure 15-18) are important for their function.

(b)    Historically, an important strategy for disrupting the adhesion of integrins to their ligands is by using a synthetic peptide that mimics the binding site on the ECM molecule to which the integrin attaches. In the case of fibronectin, the amino acid sequence is arginine-glycine-aspartate (when written using the single letter designation for each amino acid, this sequence becomes RGD). Explain why addition of such synthetic peptides would disrupt binding of cells to their normal substratum.

Figure 15-16 Fibronectin Structure. (a) A fibronectin molecule consists of two nearly identical polypeptide chains joined by two disulfide bonds near their carboxyl ends. Each polypeptide chain is folded into a series of domains linked by short, flexible segments. These domains have binding sites for ECM components or for specific receptors on the cell surface, including the tripeptide sequence RGD (arginine-glycine-aspartate), which is recognized by integrins. Besides the binding activities noted, fibronectin has binding sites for heparan sulfate, hyaluronate, and gangliosides (glycosphingolipids that contain sialic acid groups). (b) Myoblast cells on an ECM containing fibronectin in vitro, immunostained for fibronectin (green) and for DNA in the nucleus (blue).

Figure 15-18 Laminin and the Basal Lamina. (a) A laminin molecule consists of three large polypeptides—α, β, and γ—joined by disulfide bonds into a crosslinked structure. A portion of the long arm consists of a three-stranded coil. Domains on the ends of the α chain are recognized by cell surface receptors; those at the ends of the two arms of the cross are specific for type IV collagen. The cross-arms also contain laminin-laminin binding sites, which enable laminin to form large aggregates. Laminin also contains binding sites for heparin, heparan sulfate, and entactin (not shown). (Adapted with permission from Macmillan Publishers Ltd: Fig. 1 from M. P. Marinkovich, “Laminin 332 in Squamous-Cell Carcinoma,” Nature Reviews Cancer 7:370-380. Copyright 2007.) (b) Laminin assembled into a basal lamina. Laminin associates with type IV collagen, perlecan, nidogen, and other components to form a mat of extracellular matrix. Cells attach to the basal lamina using integrins.