Phosphorylation of nodulin 26 on serine 262 affects its voltage-sensitive channel activity in planar lipid bilayers

Nodulin 26 is an symbiosome membrane protein of soybean nodules that shows ion channel activity in planar lipid bilayers. Serine 262 of nodulin 26 is phosphorylated by calmodulin-like domain protein kinase. To study the effects of phosphorylation, nodulin 26 with Ser, Ala, or Asp at position 262 were expressed in Escherichia coli. The expressed protein possessed a histidine-rich leader sequence for purification by Ni2+ chelate fast protein liquid chromatography. Upon reconstitution into planar lipid bilayers, the recombinant proteins showed a large single channel conductance (3.1 nanosiemens (nS) in cis0.2M/trans1.0 M KCl and 1.6 nS in cis 0.2M/trans0.2 M KCl) and weak anion selectivity, similar to native soybean nodulin 26. Nodulin 26 with Ser- or Ala-262 occupied the maximal open conductance state greater than 97% of the time (3.1 nS in cis0.2M/trans1.0 M KCl) regardless of applied voltage. However, nodulin 26 with Asp-262 showed increased gating and preferential occupancy of lower subconductance states (1.8 and 0.6 nS in cis0.2M/trans1.0 M KCl) at high applied voltages (e.g. 70 mV). In situ phosphorylation of Ser-262 of nodulin 26 by calmodulin-like domain protein kinase also resulted in increased voltage-dependent gating and preferential occupancy of lower subconductance states. These results suggest that phosphorylation of serine 262 of nodulin 26 modulates channel activity by conferring voltage sensitivity.