Fig. 1.

Structure of the C1r-C1s heterodimer. (A) The traditional model first proposed in ref. 6. (B) Stacked tetramer model (7). Black dots indicate the positions of the binding sites for the collagen-like domains of C1q (10). (C) Gel filtration of CUB1-EGF-CUB2 fragments of C1s, C1r, and an equimolar mixture of C1r and C1s fragments in Ca²⁺ (solid line) and EDTA (black dotted line). Two different loading concentrations of C1r and C1s fragments are shown (1 and 0.5 mg/mL), and 50 µL was loaded in each case. Samples were separated on a Superdex 200 column (10/30) equilibrated in 20 mM Tris pH 7.4, containing 150 mM NaCl. Elution positions of aldolase (158 kDa), conalbumin (75 kDa), and ovalbumin (43 kDa) are shown. (D) Structure of the C1r-C1s heterodimer formed from the CUB1-EGF-CUB2 fragment of each protease. Ca²⁺ is shown as pink spheres; carbohydrates, as white sticks.