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(American Journal of Pathology. 2007;170:755-763.)
© 2007 American Society for Investigative Pathology
DOI: 10.2353/ajpath.2007.060734

Affinity of C-Reactive Protein toward FcRI Is Strongly Enhanced by the -Chain

Carlheinz Röcker^*, Dimitar E. Manolov, Elza V. Kuzmenkina^*, Kyrylo Tron^*, Holger Slatosch, Jan Torzewski and G. Ulrich Nienhaus^*

From the Departments of Biophysics^* and Internal Medicine II-Cardiology, University of Ulm, Ulm, Germany; and the Department of Physics, University of Illinois at Urbana–Champaign, Urbana, Illinois

C-reactive protein (CRP), the prototype human acute phase protein, is widely regarded as a key player in cardiovascular disease, but the identity of its cellular receptor is still under debate. By using ultrasensitive confocal imaging analysis, we have studied CRP binding to transfected COS-7 cells expressing the high-affinity IgG receptor FcRI. Here we show that CRP binds to FcRI on intact cells, with a kd of 10 ± 3 µmol/L. Transfection of COS-7 cells with a plasmid coding for both FcRI and its functional counterpart, the -chain, markedly increases CRP affinity to FcRI, resulting in a kd of 0.35 ± 0.10 µmol/L. The affinity increase results from an 30-fold enhanced association rate coefficient. The pronounced enhancement of affinity by the -chain suggests its crucial involvement in the CRP receptor interaction, possibly by mediating interactions between the transmembrane moieties of the receptors. Dissociation of CRP from the cell surfaces cannot be detected throughout the time course of several hours and is thus extremely slow. Considering the pentameric structure of CRP, this result indicates that multivalent binding and receptor clustering are crucially involved in the interaction of CRP with nucleated cells.

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