Submitted April 15, 2008
Accepted July 18, 2008
Ligation of erythrocyte CR1 induces its clustering in complex with scaffolding protein FAP-1
Ionita Ghiran*, Aleksandra M Glodek, Gregory Weaver, Lloyd B. Klickstein, and Anne Nicholson-Weller
Department of Medicine, Division of Allergy and Inflammation, Beth Israel Deaconess Medical Center, Boston, MA, United States
NIBR, Novartis, Cambridge, MA, United States
* Corresponding author; email: ighiran{at}bidmc.harvard.edu.
The primary identified function of complement receptor 1 (CR1/CD35) on primate erythrocytes is to bind complement-tagged inflammatory particles including microbes and immune complexes. When erythrocytes circulate through liver and spleen, sinusoidal phagocytes remove CR1-adherent particles and erythrocytes return to the circulation. This process of immune adherence clearance is important for host defense and prevention of autoimmunity. CR1 was previously described as clustered in the human erythrocyte membrane, which was thought to be necessary for binding complement-opsonized particles. In contrast, we demonstrate that on erythrocytes CR1 is not clustered, but dispersed, and able to bind complement-tagged particles. When fresh erythrocytes are solubilized by non-ionic detergent, CR1 partitions to the cytoskeleton fraction. Using a PDZ-peptide array, CR1's cytoplasmic tail, which contains two PDZ-motifs, binds PDZ domains 2nd, 3rd, and 5th of FAP-1 (Fas-associated phosphatase 1), a scaffolding protein. We show that FAP-1, not previously recognized as an erythroid protein, is expressed on circulating erythrocytes. CR1 and FAP-1 co-immunoprecipitate, which confirms their molecular association. Disperse CR1 on erythrocytes may be advantageous for capturing immune-complexes, while ligation-induced CR1 clustering may prevent ingestion of the erythrocyte during the immune-complex transfer to the macrophages by keeping the opsonic stimulus localized thus preventing phagocyosis.
