The 12-amino acid acidic extension interacts with the basic regions from three B-HLHZip domains: CD thermal denaturations at 222 nm using B-Max, B-Mitf, and B-Myc. (A) B-Max (closed circles), A-Max (open circles), and their mixture (open squares). The fourth curve is the sum of the B-Max and A-Max curves, a curve that is expected if the two proteins do not interact. The fact that the mixture is more stable than the Sum curve indicates that B-Max and A-Max heterodimerize. (B) B-Mitf (closed diamonds), A-Mitf (open circles), and their mixture (open squares). The fourth curve is the sum of the B-Mitf and A-Mitf curves. (C) B-Myc:B-Max (closed triangles) and B-Myc + A-Max (open squares). (D) B-Mitf (closed diamonds), A-Max (open triangles), and their mixture (open squares). The fourth curve is the sum of the B-Mitf and A-Max curves. The sum curve and the actual mixture melt superimpose, indicating that the two proteins do not interact.