Doctor of Philosophy (PhD)
Integrins are heterodimeric transmembrane proteins that play important roles in various biological processes. Most integrins serve as adhesion molecules and transmit bidirectional signals through global conformational changes. However, integrin β8 is distinctive in structure and function. Its cytoplasmic domain lacks the conserved protein-binding sequence, which is important in transmitting inside-out signals, suggesting that integrin β8 may have a different activation mechanism or lack such signaling. In addition, the ligand-binding or activating metal ion Mn2+ does not induce a global conformational change in integrin β8.
It is unclear how different domains affect integrin αvβ8 global conformation, ligand binding ability and signaling on the cell surface. In the first study, we designed cysteine mutations to determine whether the association of the αv and β8 lower legs affected its ligand binding. We showed that disulfide bonded mutants bound ligands with similar level as the wild-type protein, suggesting that αvβ8 ligand binding did not require lower leg disassociation. We further showed that the N-glycosylation mutant at the interface between the β I and hybrid domains did not affect ligand binding, suggesting that the αvβ8 open headpiece was not present on the cell surface. Our study indicates that αvβ8 integrin may adopt only one state, and the association of two lower legs might be important for stabilizing integrins in the extended conformation.
Furthermore, we designed experiments to study how different β8 domains affect its unique conformation and function. We found that the β8 EGF domains increased integrin ligand binding ability and contributed to its extended conformation. By comparison, the β8 transmembrane and cytoplasmic domains had little effect on ligand binding or global conformation. The β8 EGF and transmembrane domains did not affect integrin-mediated cell adhesion, cell spreading, focal adhesion formation or colocalization of integrin with other proteins, but the cytoplasmic domain had a defective effect on outside-in signaling. Our results showed that different domains of β8 play various roles on its unique conformation, ligand binding and signaling. The extended structure and high ligand-binding affinity of integrin αvβ8 make it ideal for encountering and binding ligands expressed on an opposing cell or in the extracellular matrix.
Song, Guannan, "Conformational Regulation and Signaling of an Atypical Integrin αvβ8" (2020). LSU Doctoral Dissertations. 5428.
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