MOLECULAR AND PHYSIOLOGICAL ANALYSIS OF A THYLAKOID K+ CHANNEL PROTEIN

Transport studies identified a K+ channel protein in preparations of p urified spinach (Spinacea oleracea) thylakoid membrane. This protein w as solubilized from native membranes and reconstituted into artificial proteoliposomes with maintenance of functional integrity. A 33-kD thy lakoid polypeptide was identified as a putative component of this thyl akoid protein. This identification was made using an antibody raised a gainst a synthetic peptide representing a highly conserved region of K + channel proteins. K+ channel activity co-migrated with the immunorea ctive 33-kD polypeptide when solubilized thylakoid membrane protein wa s fractionated on a Suc density gradient. The antibody was used to imm unoprecipitate the 33-kD polypeptide. Physiological function of this t hylakoid membrane protein was elucidated by measuring photosynthetic e lectron transport of thylakoid preparations in the presence and absenc e of a K+ channel blocker. Results indicated that K+ efflux from the t hylakoid lumen through this channel protein is required for the optimi zation of photosynthetic capacity. The effect this protein has on phot osynthetic capacity is likely due to the requirement for K+ efflux fro m the thylakoid lumen to charge-balance light-induced proton pumping a cross this membrane.