Genetic mutations surrounding the calcium in FI impair protein expression and function. (A) Linear structure of FI. FI is a multidomain glycoprotein composed of a heavy chain (50 kDa) and a light chain (38 kDa) held together by a disulfide bond (shown in yellow). The heavy chain is catalytically inactive and is composed of 4 domains (from the N-terminal) by the FI membrane attack complex (FIMAC) domain, a scavenger receptor cysteine-rich (SRCR) domain, and 2 low-density lipoprotein receptor class A (LDLRA) domains. A calcium ion (green sphere) is present in each LDLRA domain. The light chain hosts the active enzymatic site and folds into a canonical serine protease domain. (B) Mapping of 9 genetic variants (shown in cyan) on the 3-dimensional structure of FI reveals that they cluster around the calcium ion bound in LDLRA1. Note how the active site (dotted spheres) is located >40 Å away from the calcium-binding site, where the mutations cluster. (C) Table summarizing the location of each mutation and expression levels relative to wild-type (WT) measured by western blotting (WB) and enzyme-linked immunosorbent assay (ELISA). The ELISA values represent mean ± SEM from at least 3 different transfections. ∗On comparison to WT FI, the proteolytic activity of variant A258T was defective with MCP. No defect was observed with FH or complement receptor 1 (CR1) as the cofactor protein (supplemental Figure 2). n.d., not detected.