Evidence for a KCl-Stimulated, Mg2+-ATPase on the Golgi of Corn Coleoptiles

Abstract

Membranes of corn coleoptiles were fractionated by sucrose density gradient centrifugation and the locations of organelles were determined using marker enzymes and EM. Latent IDPase (or UDPase) was selected as the Golgi marker and UDPG-sterol glucosyl transferase was selected as the plasma membrane (PM) marker because they were clearly separable from markers for the other organelles. Golgi-rich and PM-rich fractions were studied in relation to their ATPase activities. The pH optimum of the KCl, Mg2+-ATPase of the PM-rich fraction from a step gradient was 6.0-6.5, while the Golgi-rich fraction had peaks at pH 6.0-6.5 and pH 7.5. The peak at pH 6.0-6.5 for the Golgi-rich fraction may be due to PM-contamination, while the peak at pH 7.5 represents the activity of a Golgi ATPase. To reduce PM contamination, Golgi-rich fractions obtained from step or rate-zonal gradients were recentrifuged isopycnically on linear sucrose gradients. The distribution of KCl, Mg2+-ATPase activity was measured at pH 6.5 and 7.5. The pH 6.5 ATPase was coincident with UDP-sterol glucosyl tranferase, a PM marker, while the pH 7.5 ATPase overlapped with latent UDPase, a Golgi marker. These results provide strong evidence for a KCl, Mg2+-ATPase, active at pH 7.5, associated with the Golgi membranes of corn coleoptiles.