Key events in the history of calcium regulation of striated muscle

Role of Proton Motive Force in Photoinduction of Cytoplasmic Streaming in Vallisneria Mesophyll Cells

In mesophyll cells of the aquatic monocot Vallisneria, red light induces rotational cytoplasmic streaming, which is regulated by the cytoplasmic concentration of Ca²⁺. Our previous investigations revealed that red light induces Ca²⁺ efflux across the plasma membrane (PM), and that both the red light-induced cytoplasmic streaming and the Ca²⁺ efflux are sensitive to vanadate, an inhibitor of P-type ATPases. In this study, pharmacological experiments suggested the involvement of PM H⁺-ATPase, one of the P-type ATPases, in the photoinduction of cytoplasmic streaming. We hypothesized that red light would activate PM H⁺-ATPase to generate a large H⁺ motive force (PMF) in a photosynthesis-dependent manner. We demonstrated that indeed, photosynthesis increased the PMF and induced phosphorylation of the penultimate residue, threonine, of PM H⁺-ATPase, which is a major activation mechanism of H⁺-ATPase. The results suggested that a large PMF generated by PM H⁺-ATPase energizes the Ca²⁺ efflux across the PM. As expected, we detected a putative Ca²⁺/H⁺ exchange activity in PM vesicles isolated from Vallisneria leaves.