Figure 4.

Agrin–LRP4 tetramer interfaces and the Ca²⁺-binding site in agrin. (A) In the LRP4 β1 propeller, the two agrin-binding interfaces (highlighted in red and yellow) are close to each other. The two bound agrin molecules are shown in the transparent cartoon. (B) A close-up view of the agrin–LRP4 dimer interface, where agrin and LRP4 are colored in blue and orange, respectively. (C) A close-up view of the agrin–agrin dimer interface, where the two agrin molecules are colored in blue and green. (D) A Ca²⁺ atom is coordinated by side chains of D1820 and D1889 and carbonyl oxygen of Q1887 and L1837. Q1887 is only 5 Å away from Q1792, which mediates the agrin–agrin dimer interface.