Fibrillin-integrin interactions in health and disease

Human fibrillin-1 is the major structural protein of extracellular matrix 10-12 nm microfibrils. it has a disulfide-rich modular organization which consists primarily of cbEGF (Ca2+-binding epidermal growth factor-like) domains and TB (transforming growth factor beta-binding protein-like) domains. TB4 contains an RGD (Arg-Gly-Asp) integrin-binding motif. The atomic structure of this region has been solved by X-ray crystallography and shows the TB4 and flanking cbEGF domains to be arranged as a tetragonal pyramid with N- and C-termini exposed at opposite ends of the fragment. The RGD integrin-binding motif is located within a flexible loop. We have used a variety of biophysical, biochemical and cell biology methods to investigate the molecular properties of integrin-fibrillin-1 interactions and have demonstrated that recombinant fibrilli-1 domain fragments mediate binding to integrins alpha V beta 3, alpha 5 beta 1 and alpha V beta 6. Integrin alpha V beta 3 is a high-affinity fibrillin-1 receptor (K-d similar to 40 nM) whereas integrins alpha V beta 6 and beta 5 beta 1 show moderate-affinity (K-d similar to 450 nM) and low-affinity (K-d > 1 mu M) binding respectively. Different patterns of alpha 5 beta 1 distribution are seen when human keratinocytes and fibroblasts are plated on to fibrillin domain fragments compared with those seen for fibronectin, suggesting that fibrillin may cause a lesser degree or different type of intracellular signalling. A number of disease-causing mutations which affect the TB4 domain have been identified. These are being investigated for their effects on integrin binding and/or changes in intramolecular structure.