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1 1Periparturient Diseases of Cattle Research Unit, USDA-ARS, National Animal Disease Center, Ames, IA, USA
2 2Bacterial Diseases of Livestock Research Unit, USDA-ARS, National Animal Disease Center, Ames, IA, USA
* To whom correspondence should be addressed. E-mail: treinhar{at}nadc.ars.usda.gov.
Based on sequence similarities to the yeast PMR1and hSPCA gene, the rat alternatively spliced mRNA has been suggested to be a Golgi secretory pathway Ca2+-ATPase (SPCA). Data in this report lends further support for this hypothesis in that sucrose gradient fractionation of rat liver microsomes resulted in SPCA co-migrating with the Golgi calcium binding protein CALNUC, which was well resolved from the endoplasmic reticulum marker calreticulin. Also, in PC-12 cells, antibody to SPCA co-localized with an antibody to the Golgi marker alpha-mannosidase II. In order to study the biological effects of SPCA expression we performed stable overexpression of SPCA in COS-7 cells. Seven clones were selected for further comparison with COS-7 cells containing an empty expression vector. Overexpression of SPCA resulted in a significant reduction of plasma membrane Ca2+-ATPase, sarco/endoplasmic reticulum Ca2+-ATPase and calreticulin expression in these clones. In contrast, the expression of the Golgi calcium-binding protein CALNUC increased significantly. The phosphoenzyme intermediate formed using membranes from clone G11/5 was calcium-dependent, significantly more intense than in COS-7 cells and not affected by La3+ treatment. Calcium uptake by G11/5 microsomes was ATP-dependent and significantly greater than in microsomes from parent COS-7 cells. The overexpression of SPCA significantly increased the growth rate of these cells compared to COS-7 cells containing only the empty vector. These data demonstrate that overexpression of the rat SPCA results in significant changes in the expression of calcium transport and storage proteins in COS-7 cells.
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