The integrin α-subunit leg extends at a Ca²⁺-dependent epitope in the thigh/genu interface upon activation

Abstract

Two activation-dependent Abs to the integrin ␣L-subunit were used to study conformational rearrangement of ␣L␤2 on the cell surface. Activation lowered the concentration of Ca 2؉ required for maximal expression of each epitope. Each Ab requires the Ca 2؉ -binding loop in the integrin genu and nearby species-specific residues in the thigh domain. Key thigh residues are shielded from Ab in the bent integrin conformation by the ␣-subunit calf-1 domain and the nearby bent ␤ leg, suggesting that extension at the genu is required for epitope exposure. Activating stimuli and ␣͞␤ I-like small molecule antagonists demonstrate that exposure of epitopes in the integrin ␣-and ␤-subunit legs is coordinate during integrin activation. A coordinating residue donated by the calf-1 domain is as important as Ca 2؉ for mAb binding. Together with inspection of the ␣V structure, this result suggests that the genu͞ calf-1 interface is maintained in integrin activation, and that extension occurs by a rearrangement at the thigh͞genu interface.

RhoA Is Involved in LFA-1 Extension Triggered by CXCL12 but Not in a Novel Outside-In LFA-1 Activation Facilitated by CXCL9

“…PMA has been demonstrated to induce LFA-1 extension measured by the LFA-1 extension epitope KIM127 (19,(31)(32)(33). Nevertheless, the short (1 min) PMA treatment, which was sufficient to potently activate LFA-1 adhesiveness (Fig.…”

AL-57, a ligand-mimetic antibody to integrin LFA-1, reveals chemokine-induced affinity up-regulation in lymphocytes

“…Activation-dependent LFA-1 mAbs reported thus far map outside of the ligand-binding inserted (I) domain: NKI-L16 (20,21) maps to the genu of the ␣ L subunit, m24 (22)(23)(24) and 327C (25) bind to the ␤ 2 I-like domain, and KIM127 (26,27) maps to I-EGF domain 2 in the ␤ 2 leg. Integrins that lack I domains have been shown to exist in at least three conformational states: bent (low affinity), extended with a closed headpiece putative intermediate affinity, and extended with an open headpiece (high affinity) (28,29).…”

NMR Spectroscopy for Studying Integrin Antagonists

“…O f p a r t i c u l a r i m p o r t a n c e i s t h e h i g h l e v e l o f s e q u e n c e conservation within the integrin family, such as the location and number of critical disulphide bonds (Luo et al, 2007). Thus, although high-resolution structures have only been obtained for two intact members of the 3 family and a single 2 integrin, the structural insights gained are applicable to other integrins (Xie et al, 2004;Xiong et al, 2001). , and extended (C) conformations.…”

“…A ring of hydrophobic residues (the hydrophobic cup) surrounds the MIDAS site (Shimaoka et al, 2002). The N-and C-termini of the I domain are in close proximity and form flexible linkers to the -propeller, as revealed in a recent crystal structure of the integrin X2 (Xie et al, 2004).…”

“…C-terminal to the -propeller is the thigh, calf-1 and calf-2 domains, all -sandwich domains that bear topological similarity with www.intechopen.com immunoglobin domains. A short disulphide bonded loop, the genu, is positioned between the thigh and calf-1 domains; the genu can form a calcium coordinating site although this is not observed in all integrins (Xie et al, 2004). A furin protease cleavage site, located in calf-2 that separates the alpha subunit into light and heavy chains is found in a subset of integrins (3, 5, 6, 8, V, and IIb) (Campbell and Humphries, 2011).…”