Fig. 2: Cryo-EM structure of human Fcμ–J–CD5L complex.

a The cryo-EM structure of the human Fcμ–J–CD5L complex is depicted in two views. Five Fcμ chains are shown in teal, and five are shown in pale cyan. The J chain, CD5L and Ca²⁺ are shown in light coral, gold and green, respectively. Consistent color schemes are applied across all figures unless specified otherwise. b Interactions between the SRCR2–SRCR3 junction of CD5L and the β5–β6 hairpin of the J chain. Polar interactions are denoted by dashed lines, whereas amino acids highlighted in blue boxes indicate CD5L sites mutated in this study. c Interactions between the SRCR3 domain of CD5L and the J chain. Ca²⁺-coordinating interactions and polar interactions involving D271, E339, and K107_J are denoted with dashed lines. d Interactions between the SRCR2 domain of CD5L and the Cμ3 of Fcμ5B. A disulfide bond forms between C191S and C414_Fcμ5B.