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The metal ion-dependent adhesion site (MIDAS) of the alphaMbeta2 integrin Mac-1 I-domain promiscuously and competitively binds multiple ligands in the regulation of Leukocyte functionThe metal ion-dependent adhesion site (MIDAS) of the alphaMbeta2 integrin Mac-1 I-domain promiscuously and competitively binds multiple ligands in the regulation of Leukocyte function
Structural highlights
Function[GP1BA_MOUSE] GP-Ib, a surface membrane protein of platelets, participates in the formation of platelet plugs by binding to the A1 domain of vWF, which is already bound to the subendothelium. Publication Abstract from PubMedCell-surface receptor interactions between leukocyte integrin macrophage-1 antigen (Mac-1, also known as CR3, alphaMbeta2, CD11b/CD18) and platelet glycoprotein Ibalpha (GPIbalpha) 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 GPIbalpha N-terminal domain (GPIbalphaN) to the locality of the integrin metal ion-dependant adhesion site (MIDAS) surface. We next determined the crystal structures of the mouse GPIbalphaN and Mac-1 I-domain to 2 A and 2.5 A resolution, respectively. The mouse Mac-1 I-domain crystal structure reveals an active conformation that is stabilized by a crystal contact from the alpha7-helix with a glutamate side chain completing the octahedral coordination sphere of the MIDAS Mg(2+) ion. The amino acid sequence of the alpha7-helix and disposition of the glutamic acid matches the C-terminal capping region alpha-helix of GPIbalpha 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 GPIbalpha leucine-rich repeat (LRR) capping alpha-helix coordinates directly to the Mac-1 MIDAS Mg(2+) ion. The Mac-1:GPIbalphaN complex involves additional interactions consolidated by an elongated pocket flanking the GPIbalphaN LRR capping alpha-helix. The GPIbalphaN alpha-helix has an HxxxE motif, which is equivalent by homology to RxxxD from the human GPIbalphaN. Subsequent mutagenesis of residues at this interface, coupled with surface plasmon resonance studies, confirmed the importance of GPIbalphaN residues H218, E222, and the Mac-1 MIDAS residue T209 to formation of the complex. Structural basis of the leukocyte integrin Mac-1 I-domain interactions with the platelet glycoprotein Ib.,Morgan J, Saleem M, Ng R, Armstrong C, Wong SS, Caulton SG, Fickling A, Williams HEL, Munday AD, Lopez JA, Searle MS, Emsley J Blood Adv. 2019 May 14;3(9):1450-1459. doi: 10.1182/bloodadvances.2018027011. PMID:31053572[1] From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine. References
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