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. 2009 May 7;113(19):4747-53.
doi: 10.1182/blood-2008-10-186551. Epub 2009 Feb 13.

Interactions of platelet integrin alphaIIb and beta3 transmembrane domains in mammalian cell membranes and their role in integrin activation

Chungho Kim  1 , Tong-Lay Lau, Tobias S Ulmer, Mark H Ginsberg
Affiliations

Interactions of platelet integrin alphaIIb and beta3 transmembrane domains in mammalian cell membranes and their role in integrin activation

Chungho Kim et al. Blood. .

Abstract

Clustering and occupancy of platelet integrin alpha(IIb)beta(3) (GPIIb-IIIa) generate biologically important signals: conversely, intracellular signals increase the integrins' affinity, leading to integrin activation; both forms of integrin signaling play important roles in hemostasis and thrombosis. Indirect evidence implicates interactions between integrin alpha and beta transmembrane domains (TMDs) and cytoplasmic domains in integrin signaling; however, efforts to directly identify these associations have met with varying and controversial results. In this study, we develop mini-integrin affinity capture and use it in combination with nuclear magnetic resonance spectroscopy to show preferential heterodimeric association of integrin alpha(IIb)beta(3) TMD tails via specific TMD interactions in mammalian cell membranes in lipid bicelles. Furthermore, charge reversal mutations at alpha(IIb)(R995)beta(3)(D723) confirm a proposed salt bridge and show that it stabilizes the TMD-tail association; talin binding to the beta(3) tail, which activates the integrin, disrupts this association. These studies establish the preferential heterodimeric interactions of integrin alpha(IIb)beta(3) TMD tails in mammalian cell membranes and document their role in integrin signaling.

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Figures

Figure 1
Figure 1
Integrin αIIb and β3 TMD-tail mini-integrins interact with each other via the TMDs. (A) Amino acid sequences of αIIbTM and β3TM used in this study. Sequences in the gray box represent amino acid residues in membrane region. GXXXG motif and GFFKR motif in αIIb are underlined. Point mutations used in this study are indicated with arrows. The previously designated membrane-proximal regions containing hydrophobic stretches in both α and β integrin are boxed. (B) Schematic diagram of TMD-tail constructs. For β3TM-TAP and αIIbTM-TAP baits, β3TM and αIIbTM (A) were fused to TAP tag for purification and an N-terminal FLAG tag for detection. Tac-αIIbTM and Tac-β3TM were made by fusion of αIIbTM and β3TM, respectively, with Tac extracellular domain. (C) CHO cells were transiently transfected with αIIbTM-TAP (bait) and Tac-αIIbTM or Tac-β3TM (preys), and cells were lysed and incubated with calmodulin beads to capture the baits. Bound Tac constructs were analyzed by Western blot using anti-Tac antibody (top panels). Expression of Tac preys (middle panel) and captured αIIbTM-TAP (bottom panel) were verified by Western blot using anti-Tac antibody and anti-FLAG antibody, respectively. The arrows indicate mature cell-surface proteins, and the arrowheads incompletely glycosylated intracellular proteins. Open symbols represent Tac-β3TM, and closed symbols represent Tac-αIIbTM. (D) CHO cells were transiently transfected with baits and preys, as indicated, and cell-surface proteins were biotinylated before cell lysis. Ten percent of the lysates were incubated with neutravidin beads to determine the input of biotinylated proteins in the lysates (middle and bottom panels). The remaining lysates were first incubated with calmodulin beads to capture the baits, and the bound proteins were eluted with 10 mM EDTA. The eluates were then incubated with NeutrAvidin beads to capture the biotinylated surface proteins and the presence of Tac preys was analyzed with Western blot using anti-Tac antibody (top panel). (E) αIIbTM-TAP constructs containing deletion of GFFKR motif or mutations of 2 Gly in GXXXG motif to Leu were tested for their binding to Tac-β3 as in (C).
Figure 2
Figure 2
In vitro αIIb-β3 TM heterodimerization. (A-C) Spectral region of TROSY-HSQC spectra showing Gly702 and Gly708 of the 2H/13C/15N-labeled β3 TM segment in the absence and presence of unlabeled αIIb TM peptide. One set of signals corresponds to the monomeric signals (A). The signal intensity ratios of monomeric-to-heterodimeric signals, but not their positions, depend on the αIIb and β3 peptide concentrations, demonstrating slow exchange kinetics on the NMR timescale between the 2 species. As evidenced by the disappearance of the monomer signals at higher peptide-to-bicelle ratios (C), heterodimerization is predominant. (D) In the presence of mutant αIIb(R995A) peptide, heterodimerization is weakened. The signal intensity ratio of heterodimer-to-monomer signals drops from 1.56 to 0.14 (C,D), and the heterodimeric signals are shifted compared with the interaction with wild-type αIIb peptide. (E) The effects of charge reversal mutations in the membrane-proximal regions of αIIb or β3 on the TMD interaction were analyzed, as in Figure 1.
Figure 3
Figure 3
Talin binding to the β3 tail inhibits the αβ TMD-tail interaction. (A) αIIbTM-TAP and Tac-β3TM were cotransfected with THD or THD(W359A) mutant into CHO cells, and their interaction was analyzed, as described in Figure 1C. Ratio of the amount of Tac-β3TM bound to αIIbTM-TAP in presence of wild-type THD to that in the presence of THD W359A is shown as a bar graph. The data are the mean ± SE of 4 experiments. (B) αIIbTM-TAP and THD were cotransfected with Tac-β3TM or Tac-β3TM(Y747A) mutant into CHO cells and their interaction was analyzed, as described in panel A. The data are mean ± SE of 3 experiments.
Figure 4
Figure 4
TMD-tail mini-integrins activate native integrin αIIbβ3. (A) Tac-αIIbTM or Tac-αIIb was transfected into CHO cells expressing αIIbβ3 (A5 cells). Twenty-four hours after transfection, cells were stained with PAC1 (activation-specific anti-αIIbβ3 antibody) and 7G7B6 (anti-Tac antibody) in the presence or absence of a selective inhibitor of ligand binding to αIIbβ3, Ro-43-5054, and then analyzed by flow cytometry to generate the dot plots shown (left). Geometric means of PAC1 binding in cells expressing different quantities of Tac constructs were plotted as larger red dots. Specific PAC1 binding was calculated by subtracting the geometric means of PAC1 binding in the presence of Ro-43-5054 from that in the absence of Ro-43-5054 in each range, and plotted as a line graph (right). (B) The effects of Tac-αIIbTMΔGFFKR and Tac-αIIbTMLXXXL on αIIbβ3 activation were analyzed, as in panel A. (C) The effects of Tac-β3TM, Tac-β3, and Tac-β3TM(Y747A) on αIIbβ3 activation were analyzed, as in panel A. The activating effects of each Tac constructs were tested at least 3 times, and representative data are shown.
Figure 5
Figure 5
αIIb TMD-tail binds to the β3 subunit of native integrin αIIbβ3. (A) αIIbTM-TAP, αIIbTMΔGFFKR-TAP, or empty vector was transfected into A5 cells, and after 24 hours, cells were lysed and incubated with calmodulin beads to capture TAP constructs. β3 was detected by Western blot using anti-β3 antibody as representative of the bound αIIbβ3 (top). Expression level of β3 in lysates (middle) and captured TAP constructs (bottom) was assessed by Western blot. (B) A5 cells transfected with Tac-αIIbTM or Tac-αIIb were detached and stained with anti-Tac antibody and PAC1. In the merged image, green and red represent Tac and active integrin, respectively. The 2.7-fold digital enlargement of areas in small rectangles is shown in insets. Bar = 5 μm. (C) A5 cells were incubated for 30 minutes at 37°C with TBS (20 mM Tris, pH 8.4, 150 mM NaCl) containing either 1 mM Ca/Mg or 5 mM EDTA, and stained with αIIbβ3 complex-specific antibody (D57). D57 binding was measured by flow cytometry and plotted as histogram. (D) A5 cells transfected with TAP constructs were detached, and surface proteins were biotinylated and treated with EDTA to dissociate αIIbβ3 complex before the cells were lysed. Five percent of the lysates were incubated with neutravidin beads to determine the input of biotinylated proteins in the lysates (lanes 5-8). The remaining lysates were first incubated with calmodulin beads to capture TAP tag, and the bound proteins were eluted with 10 mM EDTA. The eluates were further incubated with NeutrAvidin beads to capture the biotinylated protein (lanes 1-4). Anti-αIIb antibody (PMI-1), anti-β3 antibody (Rb8053), and anti-FLAG antibody (M2) were used for the Western blot, as indicated. Schematic procedure for this experiment is shown in Figure S4. (E) Model of how the αIIb TMD-tail induced αIIbβ3 activation. αIIb TMD-tail interacts with TMD-tail region of β3 in the native integrin and competes for the heterodimeric interaction between αIIb and β3, resulting in rearrangement of the TMD and activation.
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