Doctor of Philosophy (PhD)
Integrin conformational changes mediate integrin activation and signaling triggered by intracellular molecules or extracellular ligands. Even though it has been shown that TM and/or cytoplasmic α and β domains associate in the resting state and separation of these domains is required for integrin signaling, it is still not clear how this signal is transmitted from the transmembrane domain through two long extracellular legs to the ligand-binding headpiece. In addition, integrin TM homomeric association was also observed. But the role of this interaction remains elusive. In this work, the platelet integrin, αIIbβ3, has been used to elucidate the roles of integrin lower leg and TM homomeric association in integrin signalling. We first addressed whether the separation of integrin αβ lower leg is critical for integrin activation and outside-in signaling. Using a disulfide bond to restrict dissociation of the α-subunit Calf-2 domain and β-subunit I-EGF4 domain, we were able to abolish integrin inside-out activation and outside-in signaling. In contrast, disrupting the interface by introducing a glycosylation site into either subunit activated integrins for ligand binding through a global conformational change. Our results suggest that the interface of the α-subunit Calf-2 domain and β-subunit I-EGF4 domain is critical for integrin bidirectional signaling. Formation of the TM homooligomers was observed in micelles and bacterial membranes previously, and it has been proposed that this homomeric association is important for integrin activation and clustering. We then addressed whether integrin TM domains form homooligomers in mammalian cell membranes using cysteine mutagenesis scanning method. Our results show that TM homomeric interaction does not occur before or after soluble ligand binding, or during inside-out activation. In addition, even though the cysteine mutants and the heterodimeric disulfide-bounded mutant could form clusters after adhering to immobilized ligand, the integrin TM domains do not form homooligomers, suggesting that integrin TM homomeric association is not critical for integrin clustering or outside-in signaling. Therefore, the integrin TM homooligomerization is not required for integrin activation, ligand binding and signaling.
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Wang, Wei, "Signaling of integrin lower leg and transmembrane domains" (2010). LSU Doctoral Dissertations. 763.