Mutations in atypical hemolytic uremic syndrome provide evidence for the role of calcium in complement factor I

Atypical hemolytic uremic syndrome (aHUS) is a rare thrombotic microangiopathy. Genetic variants in complement proteins are found in ≈60% of patients. Of these patients, ≈15% carry mutations in complement factor I (CFI). Factor I (FI) is a multidomain serine protease that cleaves and thereby inactivates C3b and C4b in the presence of cofactor proteins. Crystal structures have shown that FI possesses 2 calcium-binding domains, low-density lipoprotein receptor class A (LDLRA) 1 and LDLRA2. Yet, the role of calcium in FI is unknown. We determined that 9 genetic variants identified in aHUS (N151S, G162D, G188A, V230E, A240G, G243R, C247G, A258T, and Q260D) cluster around the calcium-binding site of LDLRA1. Using site-directed mutagenesis, we established that the synthesis of all, except A258T, was impaired, implying defective protein folding, perhaps due to loss of calcium binding. To further explore this possibility, we generated 12 alanine mutants that coordinate with the calcium in LDLRA1 and LDLRA2 (K239A, D242A, I244A, D246A, D252A, E253A, Y276A, N279A, E281A, D283A, D289A, and D290A) and are expected to perturb calcium binding. Except for K239A and Y276A, none of the mutants was secreted. These observations suggest that calcium ions play key structural and functional roles in FI.