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

The integrin α6β4 is a receptor for laminins and provides stable adhesion of epithelial cells to the basement membranes. In addition, α6β4 is important for keratinocyte migration during wound healing and favours the invasion of carcinomas into surrounding tissue. The cytoplasmic domain of the β4 subunit is responsible for most of the intracellular interactions of the integrin; it contains four fibronectin type III domains and a Calx-β motif. The crystal structure of the Calx-β domain of β4 was determined to 1.48 Å resolution. The structure does not contain cations and biophysical data support the supposition that the Calx-β domain of β4 does not bind calcium. Comparison of the Calx-β domain of β4 with the calcium-binding domains of Na⁺/Ca²⁺-exchanger 1 reveals that in β4 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-β domain. The structure of the Calx-β domain is discussed in the context of the function and intracellular interactions of the integrin β4 subunit and a putative functional site is proposed.