UBA1 and atrial fibrillation: To determine the specificity of AF’s effect on UBA1, the effects of AF on the interactions between UBA6, another ubiquitin-activating E1, and its cognate E2 UBE2Z and between UBA2 and UBE2I (UBC9), the E1-E2 pair for catalytic SUMOylation, were investigated in a GST-UBA6 pull-down assay and anti-HA-UBE2I coIP, respectively (Fig. 3f, g).