Nodulin 26, a nodule-specific symbiosome membrane protein from soybean, is an ion channel

Nodulin 26 is an integral symbiosome membrane protein of nitrogen-fixing soybean nodules. Nodulin 26 is a member of a family of structurally homologous membrane proteins with diverse transport functions. Thus, it has been proposed to be involved in symbiosome membrane transport. Despite this claim, there has not been any evidence that nodulin 26 has a transport activity. In this study, nodulin 26 was purified from soybean nodules by a non-denaturing protocol and was reconstituted into liposomes for channel studies in planar lipid bilayers. Nodulin 26 readily incorporated into bilayers, forming single channels with a maximum unitary conductance of 3.1 nanosiemens (nS) in a recording buffer of 20 mM 3-(N-morpholino)propanesulfonate-NaOH, pH 7.4, 1 M KCl. Nodulin 26 also exhibited multiple, discreet lower conductance states ranging from 0.5 to 2.5 nS. Nodulin 26 channels were voltage-sensitive. The maximal 3.1-nS state was preferentially occupied at lower applied voltages, whereas the lower conductance states were more frequently occupied at higher voltage potentials. Nodulin 26 channels transported both cations and anions, but showed a weak selectivity for anions. These results represent the first purification and functional characterization of the nodulin 26 channel and support a role for this protein in symbiosome membrane transport.