Binding of Tamm-Horsfall protein to complement 1q and complement 1, including influence of hydrogen-ion concentration

The goal of this study was to further characterize the interaction between an abundant urinary glycoprotein, Tamm-Horsfall protein, and complement 1q to determine the robustness of this reaction under different environmental conditions (particularly pH) and to begin to determine the specificity of this reaction. The influence of pH coupled with ionic strength was evaluated with an ELISA that demonstrated immobilized Tamm-Horsfall protein bound complement 1q strongly with a KD in the nmol/L range from pH 9 to pH 5.5. Increasing the ionic strength from 10 mmol/L sodium chloride (NaCl) to 154 mmol/L NaCl decreased the affinity of Tamm-Horsfall protein for complement 1q slightly (2-7-fold) at pH 9 to pH 6.5. A resonant mirror biosensor was also utilized to evaluate the binding of Tamm-Horsfall protein to complement 1q at different pH values (pH 8.2-5.8). These studies indicated that, compared to at pH 8.2, Tamm-Horsfall protein bound complement 1q at pH 5.8 with an almost two-fold higher affinity (pH 8.2, KD = 5.1 nmol/L vs at pH 5.8, KD = 2.8 nmol/L) due to a faster association rate (pH 8.2 kass = 1.6 x 106 L/mol per s vs pH 5.8 kass = 2.9 x 106 L/mol per s). Surprisingly, the capacity of Tamm-Horsfall protein for complement 1q decreased significantly at pH 5.8, suggesting that a site for complement 1q binding to Tamm-Horsfall protein may be lost at the acidic pH. Biosensor studies also showed that Tamm-Horsfall protein bound the entire complement 1 complex with binding affinities and association rates similar to those obtained for complement 1q individually. This suggested that Tamm-Horsfall protein bound complement 1q at a site other than the region of its collagenous tail where C1r2 and C1s2 bind. By western blot analysis, it was demonstrated that Tamm-Horsfall protein bound preferentially to the C chain of complement 1q.