Fig. 1: C1ql1_gC1q undergoes the hexamer formation via domain swapping.

a Domain organization of mouse C1ql1. SP signal peptide, NT N-terminal region, CL collagen-like region, gC1q globular C1q domain, with its boundary indicated. b aSEC and SDS-PAGE analyses of the purified C1ql1_gC1q protein. c Crystal structure of C1ql1_gC1q showing a trimer and a hexamer in an asymmetric unit. The bound calcium ions (Ca²⁺) and the axis across the central Ca²⁺ (Ca²⁺-axis) in each trimer are indicated by gray spheres and blue lines, respectively. d A rotated representation of the different C1ql1_gC1q protomers in the hexamer, showing domain-swapping between two trimers through the partial unfolding of the β4-β5 hairpin. e Structural superimposition of the trimeric gC1q domain in both trimer and hexamer forms of C1ql1_gC1q. The central Ca²⁺ ions and the Ca²⁺-axis across them are indicated by colored spheres and blue lines, respectively. The major structural differences in the regions surrounding the central Ca²⁺-axis are boxed. f Three enlarged views in parallel to display the conformational changes between the classical trimer and the domain-swapped hexamer of C1ql1_gC1q. The number of Ca²⁺ ions (#1–4 from bottom to top) bound along the Ca²⁺-axis is highlighted. Key residues D214, D208, and D210, which coordinate these central Ca²⁺ ions, are indicated. In the trimer, the loop between β5 and β6 (L5-6) presents with a folded back turn, while in the hexamer, the L5-6 exhibits reversed and extended directions, and the sidechain of D208 rotates outward due to the absence of the #3 Ca²⁺ ion. g A model to illustrate the role of the central Ca²⁺-axis in regulating the flipping of the β4-β5 hairpin for the transition between the trimer and hexamer forms of C1ql1_gC1q.