Adhesive activity of Lu glycoproteins is regulated by interaction with spectrin

Abstract

The Lutheran (Lu) and Lu(v13) blood group glycoproteins function as receptors for extracellular matrix laminins. Lu and Lu(v13) are linked to the erythrocyte cytoskeleton through a direct interaction with spectrin. However, neither the molecular basis of the interaction nor its functional consequences have previously been delineated. In the present study, we defined the binding motifs of Lu and Lu(v13) on spectrin and identified a functional role for this interaction. We found that the cytoplasmic domains of both Lu and Lu(v13) bound to repeat 4 of the alpha spectrin chain. The interaction of full-length spectrin dimer to Lu and Lu(v13) was inhibited by repeat 4 of alpha-spectrin. Further, resealing of this repeat peptide into erythrocytes led to weakened Lu-cytoskeleton interaction as demonstrated by increased detergent extractability of Lu. Importantly, disruption of the Lu-spectrin linkage was accompanied by enhanced cell adhesion to laminin. We conclude that the interaction of the Lu cytoplasmic tail with the cytoskeleton regulates its adhesive receptor function.

Figure 1

Binding of recombinant spectrin fragments and spectrin single repeats to Lu(v13) and Lu. (A) Schematic presentation of recombinant spectrin fragments. The boundaries of all spectrin fragments and single repeats were defined by SMART annotations. (B) Nine GST-tagged spectrin fragments were incubated with biotin-labeled Lu(v13) peptide, and binding was detected with anti-GST antibody. Only α N-5 was brought down. (C) The GST-tagged single repeats within α N-5 fragment were incubated with biotin-labeled Lu(v13) peptide, and the binding was detected as described for panel B. Only αR4 was brought down. (D) The His-tagged single repeats were incubated with GST-tagged cytoplasmic domain of Lu, and the binding was detected with anti-His antibody. Only αR4 was brought down.

Figure 2

Inhibition of Lu-spectrin interaction by αR4. GST-tagged cytoplasmic domain of Lu was preincubated with increasing concentrations of His-tagged αR4 or His-tagged αR5 at room temperature for 30 minutes. Then the mixtures were added to the 96-well plate coated with spectrin. The binding of GST-tagged cytoplasmic domain of Lu was detected by anti-GST antibody. Note the progressive decrease of Lu binding to spectrin with the increasing concentrations of αR4 but not with αR5.

Figure 3

Interaction of spectrin or αR4 with Lu(v13) as assessed by surface plasmon resonance assay. Spectrin or GST-αR4 was immobilized onto CM5 sensor chip. Lu(v13) peptide at different concentrations (0, 0.625, 1.25, 2.5, and 5 μM) was injected at 20 μL/min over the surface in a BIAcore 3000 instrument. The figure shows dose-response curves of Lu(v13) binding to immobilized spectrin (A) or to immobilized GST-αR4 (B).

Figure 4

Immunoblot of Lu and GPC in Triton shells prepared from resealed red cell ghosts. The Triton shells were prepared from ghosts resealed without or with increasing concentrations of αR4 or αR5 as described in “Triton extraction of erythrocyte ghosts.” Proteins retained in the Triton shells were analyzed by SDS-PAGE and immunoblotted with anti-Lu and anti-GPC. Note the progressive decrease of Lu in αR4-resealed ghosts (A) but not αR5-resealed ghosts (B). GPC was unchanged in αR4-resealed ghosts (C).

Figure 5

Effect of αR4 on adhesion of red cell ghosts to laminin. (A) Red cells resealed with 40 μM GST or GST-αR4 were incubated for 1 hour at 37°C on BSA- or laminin-coated 96-well microplates. Phase-contrast images show adherent cells after filling the wells with PBS and floating the microplate upside down for 40 minutes before microscopic observation. Cells were viewed with a Zeiss LSM META 510 Confocal microscope (Zeiss, Thornwood, NY) using a lens at 10×/0.30 EC Plan-Neofluar (Zeiss). Images were collected using the Zeiss Confocal microscope laser and the Laser Scanning microscope LSM 510 version 3.2 software (Zeiss). Images were cropped using Adobe Photoshop 7.0 (Adobe Systems, San Jose, CA). (B) GST, GST-αR4, or GST-αR5 at 100 μM was introduced into red cells. Adhesion of the resealed cells to immobilized laminin was measured using the gravity-driven reverse suspension assay, described for panel A. Adhesion in the presence of GST was normalized as 1. Note the enhanced adhesion in the presence of αR4 fragment but not αR5 fragment; N = 3. (C) αR4 fragment at indicated concentrations was introduced into red cells. Adhesion was measured as described for panel B. Adhesion in the presence of 20 μM GST-α4 was normalized as 1, and the fold change was plotted against increasing concentrations of GST-α4. Note the progressively enhanced adhesion in the presence of increasing concentrations of αR4 fragment; N = 3.