Isolation and Characterization of Concanavalin A-labeled Plasma Membranes of Carrot Protoplasts

The plasma membranes of protoplasts released from carrot suspension culture cells were labeled with [(14)C]acetyl-concanavalin A. After homogenization a single labeled membrane fraction was isolated in a continuous isopycnic Renografin gradient. The labeled membranes peaked at an apparent density of 1.14 grams per cubic centimeter between the Golgi fraction at a density of 1.11 grams per cubic centimeter as determined by latent IDPase activity and the mitochondria at a density of 1.16 grams per cubic centimeter as determined by the cytochrome c oxidase activity. This method provided a very discrete peak of putative plasma membrane. On discontinuous Renografin gradients a relatively pure fraction of labeled plasma membranes could be readily isolated at the 1.122 to 1.146 grams per cubic centimeter interface. The labeled fraction was enriched in both an ATPase (pH 6.5) and a glucan synthetase with a pH optimum of 6.5 whose activity was promoted by magnesium and cellobiose. Enzyme activities were not altered by the membrane label.