Excessive activation of calmodulin kinase II (CaMKII) causes arrhythmias and heart failure, but the cellular mechanisms for CaMKII-targeted proteins causing disordered cell membrane excitability and myocardial dysfunction remain uncertain. Failing human cardiomyocytes exhibit increased CaMKII and voltage-gated Ca²⁺ channel (Ca_V1.2) activity, and enhanced expression of a specific Ca_V1.2 β-subunit protein isoform (β_2a). We recently identified Ca_V1.2 β_2a residues critical for CaMKII phosphorylation (Thr 498) and binding (Leu 493), suggesting the hypothesis that these amino acids are crucial for cardiomyopathic consequences of CaMKII signaling. Here we show WT β_2a expression causes cellular Ca²⁺ overload, arrhythmia-triggering cell membrane potential oscillations called early afterdepolarizations (EADs), and premature death in paced adult rabbit ventricular myocytes. Prevention of intracellular Ca²⁺ release by ryanodine or global cellular CaMKII inhibition reduced EADs and improved cell survival to control levels in WT β_2a-expressing ventricular myocytes. In contrast, expression of β_2a T498A or L493A mutants mimicked the protective effects of ryanodine or global cellular CaMKII inhibition by reducing Ca²⁺ entry through Ca_V1.2 and inhibiting EADs. Furthermore, Ca_V1.2 currents recorded from cells overexpressing CaMKII phosphorylation- or binding-incompetent β_2a subunits were incapable of entering a CaMKII-dependent high-activity gating mode (mode 2), indicating that β_2a Thr 498 and Leu 493 are required for Ca_V1.2 activation by CaMKII in native cells. These data show that CaMKII binding and phosphorylation sites on β_2a are concise but pivotal components of a molecular and biophysical and mechanism for EADs and impaired survival in adult cardiomyocytes.