Fig. 3: C1ql1_gC1q interacts with BAI3_eCUB in a Ca²⁺-dependent manner.

a Domain organization of human BAI3. Six fragments with indicated boundaries in the extracellular domain (ECD) of BAI3 were used to determine their binding capacities for C1ql1_gC1q by aSEC and ITC assays in the buffer containing 2 mM Ca²⁺. Consequently, the C-terminal extended CUB (eCUB) domain, which contains the typical CUB domain and the following C-terminal extension, constitutes the minimal binding region required to interact with C1ql1_gC1q. b ITC-based measurements of BAI3 boundaries binding to the C1ql1_gC1q with the representation of the fitting curves for ITC titrations. 2 mM Ca²⁺ was added to the buffer. The calculated binding affinities have been listed in (a). c–e aSEC (c, e) and ITC (d) -based analyses of BAI3_eCUB binding to the C1ql1_gC1q trimer and hexamer at a gradient concentration of Ca²⁺ in the buffer. The Ca²⁺-dependent binding of eCUB to the gC1q trimer titration was indicated by ITC-measured binding affinities across a range of Ca²⁺ concentrations under buffer conditions. f Time-course aSEC analyses are used to monitor the potential transition of the C1ql1_gC1q hexamer and trimer upon BAI3_eCUB binding under high calcium conditions.