Structure of the ANKS3-SAM L52A mutant. A) ANKS3-SAMs pack as a triple helix approximately 72 Å in diameter and 100 Å long per helical repeat. In the side view of the triple helix, two separate polymer chains are shown as cartoons (grey and black) and the third is shown as a space-filled model colored by surface electrostatics calculated using APBS in Pymol and contoured at ±1 kT/e; red is negatively charged and blue is positively charged. Looking down the polymer axis (left and right) and viewing only the surface electrostatics of a single polymer for clarity reveals the charge complementarity of the polymer surface. B) A single ANKS3-SAM homodimer is shown. Helices have been numbered 1–5. Side chains of residues found critical for the EH-and ML-surface in the negGFP binding assay are shown colored in blue and red, respectively. Both the EH- and ML-surfaces are shown colored by surface electrostatics calculated using APBS and contoured at ±1 kT/e, revealing the charge complementarity of the binding interface.