Expression, purification and spectroscopic studies of full-length Kir3.1 channel C-terminus

Chemical shift assignments of the connexin45 carboxyl terminal domain: monomer and dimer conformations

Connexin45 (Cx45) is a gap junction protein involved in cell-to-cell communication in the heart and other tissues. Here we report the (1)H, (15)N, and (13)C resonance assignments for the monomer and dimer conformations of the Cx45 carboxyl terminal (Cx45CT) domain and provide evidence of dimerization using diffusion ordered spectroscopy. The predicted secondary structure of the Cx45CT domain based on the chemical shifts identified one region of α-helical structure, which corresponds to the residues that broadened beyond detection in the dimer confirmation. Previous biophysical studies from our laboratory characterizing the CT domain from the other major cardiac connexins, Cx40 and Cx43, suggest that the amount of α-helical content may translate into the ability of a protein to dimerize. Even though the CT domain is thought to be the main regulatory domain of most connexins, the physiological role of CT dimerization is currently unknown. Therefore, these assignments will be useful for determining the intermolecular interactions that mediate Cx45CT dimerization, information that will be used to characterize dimerization in functional channels, as well as characterizing the binding sites for molecular partners involved in Cx45 regulation.

pH-dependent dimerization of the carboxyl terminal domain of Cx43

Previous studies have demonstrated that the carboxyl terminus of the gap junction protein Cx43 (Cx43CT) can act as an independent, regulatory domain that modulates intercellular communication in response to appropriate chemical stimuli. Here, we have used NMR, chemical cross-linking, and analytical ultracentrifugation to further characterize the biochemical and biophysical properties of the Connexin43 carboxyl terminal domain (S255-I382). NMR-diffusion experiments at pH 5.8 suggested that the Connexin43 carboxyl terminus (CX43CT) may have a molecular weight greater than that of a monomer. Sedimentation equilibrium and cross-linking data demonstrated a predominantly dimeric state for the Cx43CT at pH 5.8 and 6.5, with limited dimer formation at a more neutral pH. NMR-filtered nuclear Overhauser effect studies confirmed these observations and identified specific areas of parallel orientation within Cx43CT, likely corresponding to dimerization domains. These regions included a portion of the SH3 binding domain, as well as two fragments previously found to organize in alpha-helical structures. Together, these data show that acidification causes Cx43CT dimer formation in vitro. Whether dimer formation is an important structural component of the regulation of Connexin43 channels remains to be determined. Dimerization may alter the affinity of Cx43CT regions for specific molecular partners, thus modifying the regulation of gap junction channels.