The 'metabolon,' CD47, and the 'phagocytic synapse': molecular co-localization and species divergence.
CD47 is a widely expressed integral membrane protein, found also on red blood cells where it reportedly has a key role in inhibiting phagocytic clearance of RBC by signaling within a multi-molecular 'phagocytic synapse'. Calreticulin is postulated to be on the RBC surface and stimulate phagocytosis, whereupon CD47 on the RBC binds SIRPalpha on the phagocyte and signals a block against phagocytosis. While studies of mouse suggest such an inhibitory role for CD47, CD47 seems to have distinct interactions in human RBC--particularly within a 'metabolon' complex of CD47, Rh proteins, and several other proteins. We have assessed the relative density, co-clustering, and mobility of some of the implicated proteins on human RBC versus murine RBC (hu-RBC and mu-RBC, respectively), and we find a few major differences. While RBC from both species express similar densities of CD47 and SIRPalpha interactions are measurably modest, the interactions prove species-specific. While RBC from both species also have detectable calreticulin, fresh hu-RBC are found to have 10-100-fold more calreticulin binding sites on their surface. Imaging of clusters of SIRPalpha-CD47 on both species of RBC show that RhD does co-localize with CD47 on hu-RBC, but neither calreticulin nor Glycophorin-A appear enriched in the metabolon complexes. Furthermore, mouse-cells alone tend to aggregate due to cross-bridging by SIRPalpha complexes, showing accumulation of CD47 in the adhesion zone, which is consistent with a high mobility of CD47 unique to mu-RBC.
Subramanian
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S
Institute for Medicine and Engineering, University of Pennsylvania, Philadelphia, PA 19104, USA.
Tsai
R
R
Discher
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DE
D000818:Animals; D001457:Anion Exchange Protein 1, Erythrocyte / Q000378:metabolism; D000943:Antigens, Differentiation / Q000378:metabolism; D001798:Blood Proteins / Q000378:metabolism*; D051928:CD47 Antigen / Q000032:analysis / Q000502:physiology*; D019556:COS Cells; D037282:Calreticulin / Q000037:antagonists & inhibitors / Q000097:blood; D002522:Chlorocebus aethiops; D004910:Erythrocyte Membrane / Q000737:chemistry* / Q000502:physiology; D006021:Glycophorins / Q000378:metabolism; D006801:Humans; D008562:Membrane Glycoproteins / Q000378:metabolism*; D051379:Mice; D008856:Microscopy, Fluorescence; D008954:Models, Biological; D046912:Multiprotein Complexes; D010587:Phagocytosis / Q000502:physiology; D011971:Receptors, Immunologic / Q000235:genetics / Q000378:metabolism; D011993:Recombinant Fusion Proteins / Q000378:metabolism; D012204:Rh-Hr Blood-Group System / Q000378:metabolism*; D012995:Solubility; D013045:Species Specificity
D001457:Anion Exchange Protein 1, Erythrocyte; D000943:Antigens, Differentiation; D001798:Blood Proteins; D051928:CD47 Antigen; C497894:CD47 protein, human; D037282:Calreticulin; C497895:Cd47 protein, mouse; D006021:Glycophorins; D008562:Membrane Glycoproteins; D046912:Multiprotein Complexes; C501922:Sirpa protein, mouse; C498523:RHAG protein, human; D011971:Receptors, Immunologic; D011993:Recombinant Fusion Proteins; D012204:Rh-Hr Blood-Group System; C026410:Rho(D) antigen; C501920:SIRPA protein, human