Ion channel activity of transmembrane segment 6 of Escherichia coli proton-dependent manganese transporter.

Synthetic peptides corresponding to the sixth transmembrane segment (TMS6) of secondary-active transporter MntH (Proton-dependent Manganese Transporter) from Escherichia coli and its two mutations in the functionally important conserved histidine residue were used as a model for structure-function study of MntH. The secondary structure of the peptides was estimated in different environments using circular dichroism spectroscopy. These peptides interacted with and adopted helical conformations in lipid membranes. Electrophysiological experiments demonstrated that TMS6 was able to form multi-state ion channels in model biological membranes. Electrophysiological properties of these weakly cation-selective ion channels were strongly dependent on the surrounding pH. Manganese ion, as a physiological substrate of MntH, enhanced the conductivity of TMS6 channels, influenced the transition between closed and open states, and affected the peptide conformations. Moreover, functional properties of peptides carrying two different mutations of His(211) were analogous to in vivo functional characteristics of Nramp/MntH proteins mutated at homologous residues. Hence, a single functionally important TMS can retain some of the functional properties of the full-length protein. These findings could contribute to understanding the structure-function relationship at the molecular level. However it remains unclear to what extent the peptide-specific channel activity represents a functional aspect of the full-length membrane carrier protein.