α2 integrin I domain mediates adhesion to C1q. (A-B) The binding of the integrin I domain and GST to type 1 collagen (A) or C1q (B) was measured in a solid-phase binding assay. (C) The binding of the α₂β₁ integrin I domain, D151A α₂β₁ integrin I domain mutant, D254A α₂β₁ integrin I domain mutant, and GST to C1q was measured in a solid-phase binding assay. (D) The binding of the α₂ integrin I domain, E318A α₂ integrin I domain mutant, and GST to C1q was measured in a solid-phase binding assay. All experiments were carried out in the presence of 2 mM MgCl₂. All results are presented as mean ± SEM from triplicate wells of a single experiment and represent 1 of at least 3 experiments demonstrating similar results. In all cases, at least 2 different preparations of purified I domain were used to verify that any results were not unique to a particular protein preparation.