Abstract
Mechanisms underlying the initiation and persistence of lethal cardiac rhythms are of significant clinical and scientific interests. Gap junctions are principally involved in forming the electrical connections between myocytes, and changes in distribution, density, and properties are consistent characteristics in arrhythmic heart disease. Therefore, understanding the structure and function of gap junctions during normal and abnormal impulse propagation are essential in the control of arrhythmias. For example, Cx45 is predominately expressed in the specialized myocytes of the impulse generation and conduction system. In both ventricular and atrial human working myocytes, Cx45 is present in very low quantities. However, a reduction in Cx43 coupled with an increased Cx45 protein levels within the ventricles have been observed after myocardial infarction and end-stage heart failure. Cx45 may influence electrical and/or metabolic coupling as a result of pathophysiological overexpression. Our goal was to identify mechanisms that could cause cellular coupling to be different between the cardiac connexins. Based upon the conserved transmembrane and extracellular loop segments, our focus was on identifying features within the divergent cytoplasmic portions. Here, we biophysically characterize the carboxyl-terminal domain of Cx45 (Cx45CT). Purification revealed the possibility of oligomeric species, which was confirmed by analytical ultracentrifugation experiments. Sedimentation equilibrium and circular dichroism studies of different Cx45CT constructs identified one region of α-helical structure (A333-N361) that mediates CT dimerization through hydrophobic contacts. Interestingly, the binding affinity of Cx45CT dimerization is 1000-fold stronger than Cx43CT dimerization. Cx45CT resonance assignments were also used to identify the binding sites and affinities of molecular partners involved in the Cx45 regulation; although none disrupted dimerization, many of these proteins interacted within one intrinsically disordered region (P278-P285). This domain has similarities with other cardiac connexins, and we propose they constitute a master regulatory domain, which contains overlapping molecular partner binding, cis-trans proline isomerization, and phosphorylation sites.