The regulation of integrin function by divalent cations

Abstract

Integrins are a family of α/β heterodimeric adhesion metalloprotein receptors and their functions are highly dependent on and regulated by different divalent cations. Recently advanced studies have revolutionized our perception of integrin metal ion-binding sites and their specific functions. Ligand binding to integrins is bridged by a divalent cation bound at the MIDAS motif on top of either α I domain in I domain-containing integrins or β I domain in α I domain-less integrins. The MIDAS motif in β I domain is flanked by ADMIDAS and SyMBS, the other two crucial metal ion binding sites playing pivotal roles in the regulation of integrin affinity and bidirectional signaling across the plasma membrane. The β-propeller domain of α subunit contains three or four β-hairpin loop-like Ca(2+)-binding motifs that have essential roles in integrin biogenesis. The function of another Ca(2+)-binding motif located at the genu of α subunit remains elusive. Here, we provide an overview of the integrin metal ion-binding sites and discuss their roles in the regulation of integrin functions.

Figure 1. Integrin family. Integrins are loosely grouped into three classes that bind basal extracellular matrix (ECM), provisional ECM and cell surface adhesion molecules (CAMs), respectively. Basal ECM mainly includes collagen and laminin. Provisional ECM mainly includes fibrinogen, fibronectin, vitronectin, cryptic collagen and von Willebrand factor. The α I-containing integrins are asterisked.

Figure 2. Schematic of integrin structure and conformational rearrangements. (A) Organization of domains within the primary structure. α I domain inserted in β-propeller is denoted by dash lines. Yellow and red asterisks denote Ca²⁺- and Mg²⁺-binding sites, respectively. Open asterisk denotes the Ca²⁺-binding site in the forth repeat of β-propeller domain in some α subunits. (B and C) Conformational rearrangements of α I-containing (B) and α I-less integrins (C) during activation.

Figure 3. Ca²⁺-binding sites in α subunit β-propeller domain. (A) Sequence alignment of the Ca²⁺-binding sites in β-propeller domain of 18 human α subunits. The α subunits containing α I domain are asterisked. Residues with metal-coordinating side-chain oxygen atoms are highlighted in red, and residues with metal-coordinating backbone carbonyl oxygen atoms are highlighted in deep purple. The residue numbers are shown in light blue on the left of the first residue of each Ca²⁺ binding sequence. (B) Side-view of the unliganded crystal structure of αIIbβ3 headpiece (pdb3FCS), with αIIb subunit shown in green and β3 in blue. The αIIb β-propeller bound Ca²⁺ ions are denoted in yellow spheres. (C) Schematic of the αIIb β-propeller domain showing the central cage motif (top view). (D) Schematic of the “W” topology of blade 4 (side view) in αIIb β-propeller domain. The relative locations of the seven mutations found in Glanzmann thrombasthenia patients are shown. The mutant residues are shown in small red spheres and the bound Ca²⁺ are shown in a large yellow sphere.

Figure 4. Structural rearrangements of integrin α/β I domains and their metal ion-binding sites. (A–C) αM I domain and MIDAS. (A) Superposition of integrin αM I domains in low-affinity (pdb1JLM, cyan) and high-affinity (pdb1IDO, green) conformations. Blue and red spheres denote the MIDAS metal ions in low- and high-affinity conformations, respectively. (B and C) MIDAS from the closed (pdb1JLM) (B) and open (pdb1IDO) (C) αM I domains. The metal ion coordinations are shown by red dashed lines. Glu314 from a neighboring αM I domain in crystal lattice coordinates with the MIDAS Mg²⁺ and is shown in yellow. Large blue and red spheres are Mn²⁺ and Mg²⁺, respectively, and small red spheres are coordinating water-molecule oxygens. (D–F) β3 I domain and its metal ion cluster. (D) Superposition of β subunit I domains from integrin αIIbβ3 in low-affinity (pdb3FCS, cyan) and high-affinity (pdb3FCU, green) conformations. Blue and red spheres denote Mg²⁺ in low- and high-affinity MIDAS, respectively. Orange and yellow spheres denote Ca²⁺ in low- and high-affinity β I domains, respectively. (E) Structure of low-affinity β3 metal ion-binding sites (pdb3FCS). (F) Structure of high-affinity β3 metal ion-binding sites (pdb3FCU). (G) Structure of metal ion-binding sites in low-affinity β2 (pdb3K6S). The linear cluster of β I domain metal ion-binding sites are shown as SyMBS, MIDAS and ADMIDAS from left to right. The metal ion-binding sites are colored as follows: yellow, SyMBS; pink, MIDAS; purple-blue, ADMIDAS. Mg²⁺ ions in low- and high-affinity MIDAS are shown as large blue and red spheres, respectively. The SyMBS and ADMIDAS bound Ca²⁺ ions are shown as large yellow spheres. Coordinating water-molecule oxygens are shown as small red spheres. N and O atoms involved in metal ion-coordinating are colored in blue and red, respectively. Coordinations between O atoms and metal ions are shown by red dashed lines. The cation-π interaction between the aromatic side chain of Y164 (in β3) and SyMBS metal ion is shown by blue dashed line.