IDENTIFICATION, KINETIC-PROPERTIES AND INTRACELLULAR-LOCALIZATION OF THE (CA2+-MG2+)-ATPASE FROM THE INTRACELLULAR STORES OF CHICKEN CEREBELLUM

The microsomal fraction of chicken cerebellum expresses a large amount of Ca2+-ATPase (105 kDa), which is phosphorylated by ATP in the presence of Ca2+. The Ca2+-ATPase activity is highly sensitive to temperature and to the presence of detergents. This ATPase has kinetic properties similar to those of chicken skeletal-muscle sarcoplasmic reticulum, as (i) it is activated by low (mu-M) and inhibited by high (mM) Ca2+ concentrations, (ii) it shows biphasic activation with ATP and (iii) it is inhibited by vanadate. However, the vanadate-sensitivity is at least 10 times greater than that observed in chicken skeletal or cardiac sarcoplasmic-reticulum Ca2+-ATPases. Thus, despite cross-reacting with antibodies against the cardiac and skeletal isoforms, the cerebellar microsomal Ca2+-ATPase appears to be distinct from both muscle enzymes. The Ca2+-ATPase is concentrated in, but not exclusive to, Purkinje neurons. In Purkinje neurons the Ca2+-ATPase appears to be expressed throughout the cell body, the dendritic tree (and the spines) and the axons. At the electron-microscope level the Ca2+-ATPase is found in smooth and rough endoplasmic-reticulum cisternae as well as in other, yet unidentified, smooth-surfaced structures.