The solution structure of EMILIN1 globular C1q domain reveals a disordered insertion necessary for interaction with the alpha4beta1 integrin

The extracellular matrix protein EMILIN1 (elastin microfi- bril interface located protein 1) is implicated in maintaining blood pressure homeostasis via the N-terminal elastin microfi- bril interface domain and in trophoblast invasion of the uterine wall via the globular C1q (gC1q) domain. Here, we describe the first NMR-based homology model structure of the human 52-kDa homotrimer of the EMILIN1 gC1q domain. In contrast to all of the gC1q (crystal) structures solved to date, the 10-stranded ␤-sandwich fold of the gC1q domain is reduced to nine ␤ strands with a consequent increase in the size of the cen- tral cavity lumen. An unstructured loop, resulting from an inser- tion unique to EMILIN1 and EMILIN2 family members and located at the trimer apex upstream of the missing strand, spe- cifically engages the ␣4␤1 integrin. Using both Jurkat T and EA.hy926 endothelial cells as well as site-directed mutagenesis, we demonstrate that the ability of ␣4␤1 integrins to recognize the trimeric EMILIN1 gC1q domain mainly depends on a single glutamic acid residue (Glu933). Static and flow adhesion of T cells and haptotactic migration of endothelial cells on gC1q is fully dependent on this residue. Thus, EMILIN1 gC1q-␣4␤1 represents a unique ligand/receptor system, with a requirement for a 3-fold arrangement of the interaction site.