Structural Studies of Integrin Activation
Nicholas Jay Anthis
Submitted for the degree of Doctor of Philosophy
Lincoln College, University of Oxford, Trinity Term 2009
Fundamental to cell adhesion and migration, integrins are large heterodimeric membrane proteins that link the extracellular matrix to the actin cytoskeleton. Uniquely, these adhesion receptors mediate inside-out signal transduction, whereby extracellular adhesion is activated from within the cell by talin, a large cytoskeletal protein that binds to the cytoplasmic tail of the β integrin subunit via its PTB-like F3 domain. Features of the interface between talin1 and small β3 fragments only have been described previously.
Through NMR studies of full-length integrin β tails, we have found that β tails differ widely in their interactions with different talin isoforms. The muscle-specific β1D/talin2 complex exhibited particularly high affinity, leading to the X-ray crystal structure of the β1D tail/talin2 F2-F3 complex. Further NMR and biological experiments demonstrated that integrin activation is induced by a concerted series of interactions between the talin F3 domain and the β tail and between the talin F2 domain and the cell membrane. Additional studies revealed the structural determinants of tight talin2/β1D binding and the basis of more general differences between β1 and β3 talin binding.
NMR studies were also performed on tyrosine-phosphorylated integrin tails binding to the PTB domains of talin1 and Dok1, an inhibitor of integrin activation; these revealed that phosphorylation can inhibit integrin activation by increasing the affinity of the β tail for talin competitors. Key residues governing this switch were identified, and proteins were engineered with reversed affinities, offering potentially useful biological tools. Taken together, these results reveal the remarkable complexity of structural features that enable talin and its competitors to mediate this important form of transmembrane signalling.