Structure of the Calx-β domain of the integrin β4 subunit: insights into function and cation-independent stability

The integrin alpha 6 beta 4 is a receptor for laminins and provides stable adhesion of epithelial cells to the basement membranes. In addition, alpha 6 beta 4 is important for keratinocyte migration during wound healing and favours the invasion of carcinomas into surrounding tissue. The cytoplasmic domain of the beta 4 subunit is responsible for most of the intracellular interactions of the integrin; it contains four fibronectin type III domains and a Calx-beta motif. The crystal structure of the Calx-beta domain of beta 4 was determined to 1.48 angstrom resolution. The structure does not contain cations and biophysical data support the supposition that the Calx-beta domain of beta 4 does not bind calcium. Comparison of the Calx-beta domain of beta 4 with the calcium-binding domains of Na+/Ca2+-exchanger 1 reveals that in beta 4 Arg1003 occupies a position equivalent to that of the calcium ions in the Na+/Ca2+-exchanger. By combining mutagenesis and thermally induced unfolding, it is shown that Arg1003 contributes to the stability of the Calx-beta domain. The structure of the Calx-beta domain is discussed in the context of the function and intracellular interactions of the integrin beta 4 subunit and a putative functional site is proposed.