Binding of human C-reactive protein (CRP) to plasma fibronectin occurs via the phosphorylcholine-binding site

Human C-reactive protein (CRP) is an acute phase reactant that is selectively deposited at sites of tissue damage. CRP binds with high affinity to purified plasma fibronectin (Fn) when the Fn is immobilized on a surface or matrix via either specific IgG antibody or by gelatin. The CRP to Fn binding is saturable at a molar ratio of CRP/Fn of approximately 9 with a Kd = 1.47 x 10(-7) M and requires Ca2+. The binding site on Fn for CRP was localized to the C-terminal portion by using monoclonal antibodies (mAbs) to Fn as competitive inhibitors of CRP. The binding involves the phosphorylcholine (PC)-binding site of CRP since the addition of PC inhibits binding to Fn and those mAbs to CRP that bind at or near the PC-binding site selectively inhibit the CRP to Fn binding. In addition the mouse IgA myeloma protein TEPC-15, which is specific for PC, also competes with CRP for binding sites on Fn. A mAb to the mouse PC-binding idiotype T-15, which also reacts with the PC-binding site of CRP, inhibits the binding of CRP to Fn. The findings suggest that CRP may play a role in the formation of the extracellular matrix needed for tissue repair. The CRP-Fn interaction may be one of the explanations for the observation of selective deposition of CRP at sites of tissue injury.