Mechanistic Insights into αIIbβ3 Integrin Signaling from Long-Timescale Molecular Dynamics Simulations

Abstract

Integrin αIIbβ3 is a heterodimeric cell adhesion receptor composed of α and β subunits, each of which features an extracellular domain (ECD), a single transmembrane (TM) α-helix, and an intracellular cytoplasmic tail (CT). Like other integrins, αIIbβ3 can signal bi-directionally through interaction of its ECD with extracellular ligands (“outside-in” signaling) or its CT with intracellular proteins (“inside-out” signaling). Notwithstanding the recent developments in structural biology of integrins, questions remain about the molecular determinants that are responsible for protein activation and signaling. To obtain rigorous mechanistic insights into αIIbβ3 “inside-out” signaling at an unprecedented level of molecular detail, we carried out microsecond-scale all-atom molecular dynamics (MD) simulations of various experimentally-based three-dimensional models of the TM and CT regions of αIIbβ3 in an explicit 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocoline bilayer, and in the presence or absence of the F2-F3 subdomains of the intracellular activator talin-1.These simulations broaden our current understanding of the mechanism of αIIbβ3 activation by talin-1. Specifically, we observe: a) different interactions between the αIIb and β3 CTs with or without the F2-F3 subdomains of talin-1; b) a modulatory effect of F2-F3 on the αIIbβ3 TM and CT structures, c) specific electrostatic interactions between talin-1 F2-F3 and the phospholipid polar groups; and d) flexibility of the linker region between the F3 and F2 resulting in their re-orientation with respect to the corresponding crystal structure. Notably, these simulations reveal novel stable interactions between residues of the αIIb and β3 CTs (E1005-K725, E1006-K729, and E1008-R736, respectively), the αIIb CT and the talin-1 F3 (E1001-K316 and D1004-K364, respectively), as well as the β3 CT and the talin-1 F2-F3 (E726-K316, E726-Q381, T753-N355, F754-Y373, and R760-E293, respectively), the mutation of which may be worthy of experimental testing based on their expected interference with αIIbβ3 activation.