Alanine mutagenesis of residues in LDLRA1 and LDLRA2. (A) Side chain of residues (shown in cyan) surrounding the 2 calcium ions (green spheres) selected for the alanine mutagenesis studies. (B) Table summarizing the location of each mutation and expression levels relative to WT measured by WB and ELISA. The ELISA values represent mean ± SEM from at least 3 different transfections. ∗On comparison to WT FI, the proteolytic activity of variant K239A was defective with MCP, whereas no defect was observed with FH or CR1 as the cofactor protein (supplemental Figure 4). n.d., not detected.