(A) β_IV-spectrin organization in WT and qv^3J animals. qv^3J animals lacked a CaMKII-binding domain, but retained actin-, ankyrin-, and α-spectrin–binding domains. (B and C) Schematic of control GST–β_IV-spectrin fusion protein encompassing spectrin repeats 13–17 and the CaMKII-binding domain (βIV-WT) and truncated mutant lacking CaMKII-binding domain (βIV-qv^3J). (D) β_IV-spectrin WT and qv^3J GST fusion proteins were incubated with detergent-soluble rat heart lysate and analyzed by immunoblot (CaMKIIδ). L-type Ca²⁺ channel β2a subunit was also expressed as a GST fusion protein and used as positive control. CaMKIIδ bound to the β2a and WT GST fusion proteins, but not to the qv^3J GST fusion protein. Lanes were run on the same gel but were noncontiguous (white line). (E) β_IV-spectrin WT and qv^3J GST fusion proteins retained binding activity for ankyrin-G. (F) Coimmunoprecipitation studies showing association of ankyrin-G and CaMKIIδ in WT, but not qv^3J, hearts. (G) Expression of ankyrin-G, Na_v1.5, CaMKIIδ, β-catenin, and N-cadherin in heart lysates from WT and qv^3J animals. Actin is shown as loading control. (H–Q) Permeabilized adult rat cardiomyocytes from (H–L) WT and (M–Q) qv^3J hearts were immunostained for (H and M) N-cadherin, (I and N) total CaMKIIδ, (J and O) CaMKII-phospho-T287, (K and P) β_IV-spectrin, and (L and Q) Na_v1.5. Localization of CaMKIIδ (total and phospho-T287) to the intercalated disc (white arrows) was disrupted in qv^3J cardiomyocytes. Scale bars: 10 μm (H–Q).