Calcium transport by pea root membranes : II. Effects of calmodulin and inhibitors

Calcium transport by purified endomembrane vesicles from roots of Pisum sativum L. was studied. Two types of ATP-dependent transport were demonstrated, protonophore-sensitive calcium/proton antiport and protonophore-insensitive transport. The former was predominantly located in the tonoplast, while the latter was associated with the plasma membrane and Golgi apparatus. Calcium uptake by membranes of low buoyant density (tonoplast) was inhibited by nitrate and was not sensitive to sodium orthovanadate while that by membranes of high buoyant density (plasma membrane, Golgi apparatus) was inhibited by sodium orthovanadate and insensitive to nitrate. Endoplasmic reticulum also took up calcium in the presence of ATP; however, because of its imperfect separation from the tonoplast, the nature of the calcium-transport system present could not be fully characterised. None of the calcium transporters were stimulated by calmodulin or inhibited by calmodulin inhibitors.