Structure and Organization of the N2A region and S100A1 . (A) Schematic of the structural domains in the N2A region. The N2A region consists of four Ig domains (I80, I81, I82, and I83) and a unique insertion sequence (UN2A) with a tri-helix bundle at its core and disordered termini (dashed lines). (B) Schematic of the tri-helix bundle in UN2A. (C) Schematic of the Ca²⁺-binding protein, S100A1. S100A1 exists as a homodimer in its native state and binds to four Ca²⁺ ions (green spheres).