Structural basis of the leukocyte integrin Mac-1 I-domain interactions with the platelet glycoprotein Ib.

ABSTRACT: Cell-surface receptor interactions between leukocyte integrin macrophage-1 antigen (Mac-1, also known as CR3, ?M?2, CD11b/CD18) and platelet glycoprotein Ib? (GPIb?) are critical to vascular inflammation. To define the key residues at the binding interface, we used nuclear magnetic resonance (NMR) to assign the spectra of the mouse Mac-1 I-domain and mapped the residues contacting the mouse GPIb? N-terminal domain (GPIb?N) to the locality of the integrin metal ion-dependant adhesion site (MIDAS) surface. We next determined the crystal structures of the mouse GPIb?N and Mac-1 I-domain to 2 Å and 2.5 Å resolution, respectively. The mouse Mac-1 I-domain crystal structure reveals an active conformation that is stabilized by a crystal contact from the ?7-helix with a glutamate side chain completing the octahedral coordination sphere of the MIDAS Mg2+ ion. The amino acid sequence of the ?7-helix and disposition of the glutamic acid matches the C-terminal capping region ?-helix of GPIb? effectively acting as a ligand mimetic. Using these crystal structures in combination with NMR measurements and docking analysis, we developed a model whereby an acidic residue from the GPIb? leucine-rich repeat (LRR) capping ?-helix coordinates directly to the Mac-1 MIDAS Mg2+ ion. The Mac-1:GPIb?N complex involves additional interactions consolidated by an elongated pocket flanking the GPIb?N LRR capping ?-helix. The GPIb?N ?-helix has an HxxxE motif, which is equivalent by homology to RxxxD from the human GPIb?N. Subsequent mutagenesis of residues at this interface, coupled with surface plasmon resonance studies, confirmed the importance of GPIb?N residues H218, E222, and the Mac-1 MIDAS residue T209 to formation of the complex.

SUBMITTER: Morgan J 

PROVIDER: S-EPMC6517656 | biostudies-literature | 2019 May

REPOSITORIES: biostudies-literature