Activation of 1,3‐β‐glucan synthase by Ca2+, spermine and cellobiose. – Localization of activator sites using inside‐out plasma membrane vesicles

The plant plasma membrane contains a 1,3‐β‐glucan synthase (EC 2.4.1.34) which has its active site on the cytoplasmic side of the membrane, while the product, the cell wall component callose, is deposited on the apoplastic side of the membrane. This enzyme should therefore be an integral, transmembrane protein. The activity of the enzyme is stimulated by Ca2+, polyamines, and polyols. Using sealed, inside‐out (cytoplasmic side out) plasma membrane vesicles from sugar beet (Beta vulgaris L.) leaves, which permits the activity to be measured without solubilization of the membrane, we have localized the activator sites for Ca2+, spermine, and cellobiose to the cytoplasmic side of the plasma membrane.