Molecular and Physiological Analysis of a Thylakoid K+ Channel Protein

Transport studies identified a K+ channel protein in preparations of purified spinach (Spinacea oleracea) thylakoid membrane. This protein was solubilized from native membranes and reconstituted into artificial proteoliposomes with maintenance of functional integrity. A 33-kD thylakoid polypeptide was identified as a putative component of this thylakoid protein. This identification was made using an antibody raised against a synthetic peptide representing a highly conserved region of K+ channel proteins. K+ channel activity co-migrated with the immunoreactive 33-kD polypeptide when solubilized thylakoid membrane protein was fractionated on a Suc density gradient. The antibody was used to immunoprecipitate the 33-kD polypeptide. Physiological function of this thylakoid membrane protein was elucidated by measuring photosynthetic electron transport of thylakoid preparations in the presence and absence of a K+ channel blocker. Results indicated that K+ efflux from the thylakoid lumen through this channel protein is required for the optimization of photosynthetic capacity. The effect this protein has on photosynthetic capacity is likely due to the requirement for K+ efflux from the thylakoid lumen to charge-balance light-induced proton pumping across this membrane.