Recombinant integrin ectodomains
To study integrin-matrix interactions, we have generated ectodomain heterodimers, which associate via the heterodimerizing leucine zippers of the Fos and Jun transcription factors. These soluble integrins are mainly produced in insect cell expression systems and isolated with high purity and activity by ligand affinity chromatography [1, 2].
The soluble integrins have been instrumental:
- to study integrin-matrix interactions at the molecular level in a cell-free binding assay, such as titration ELISAs [2-12], for which a novel evaluation algorithm has been developed . Molecular forces between individual integrin molecules and their ligands can be measured by biointerface force spectroscopy [14, 15] or by atomic force microscopy [3, 6]. Moreover, individual molecules of soluble integrins are visualized and their different conformation can be distinguished by electron microscopy .
- to test integrins as potential docking sites for pathogens and vehicles for gene therapy [16-20].
- To map integrin binding sites in collagens and other extracellular matrix molecules [21-23].
- to screen venoms for potential and novel integrin inhibitors [9, 11, 24-40]
- to characterize integrin-specific cell functions [3-6, 8, 19, 21, 23, 41-67].
Similarly to the soluble integrins, recombinantly expressed domains of collagens and of laminin-332 provide insights in the integrin-mediated cell-matrix interactions. In contrast to the RGD-peptide motif, the integrin recognition sites in collagens and laminins depend on the quaternary structure and three-dimensional conformation. The triple helix of collagens is essential for integrin binding [6, 10, 21, 22, 68-71]. Moreover it determines the substrate specificity of collagenases, making the triple helix an essential prerequisite to study collagenase functions [35, 39, 72-75]. To generate small size collagenous triple helices, the foldon trimerization domain of a bacteriophage was instrumental for their recombinant expression as trimeric molecules in bacteria . Such foldon-stabilized mini-collagens harboring an integrin binding site are soluble molecules, in contrast to matrix-deposited collagens, and enabled to identify integrin-specific signaling events in collagen-induced platelet activation .
The coiled-coil-stabilized G-domain of laminin-332, as well as the C-terminal sequence of the laminin are key elements for integrin binding. Only trimeric mini-laminins allow integrin-mediated cell reactions [46, 60, 61, 76-79].
Moreover, the mini-integrin-ligands are suitable antigens to produce functional antibodies for the analysis of integrin-ligand interactions.
The mini-integrin ligands has been used to measure mechanical forces between individual integrins and their ligands [4, 6, 14, 15].