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Calnuc, an EF-Hand Ca²⁺-Binding Protein, Is Stored and Processed in the Golgi and Secreted by the Constitutive-Like Pathway in AtT20 Cells

Departments of Cellular and Molecular Medicine and Pathology, University of California San Diego, La Jolla, California 92093-0651

Address all correspondence and requests for reprints to: Marilyn G. Farquhar, Ph.D., Department of Cellular & Molecular Medicine, University of California San Diego, CMM-West, Room 210, 9500 Gilman Drive, La Jolla, California 92093-0651. E-mail: mfarquhar{at}ucsd.edu.

Calnuc is an ubiquitous, EF-hand Ca²⁺ binding protein found in the cytoplasm where it binds to Gi3, in the Golgi lumen where it constitutes a Ca²⁺ storage pool, and secreted outside the cell. Here we investigated the pathway of secretion of calnuc in AtT20 cells. We found by pulse-chase experiments that calnuc is synthesized in the endoplasmic reticulum, transported to the Golgi where it remains greater than 12 h and undergoes posttranslational modification (O-glycosylation and sulfation) followed by secretion into the culture medium. We examined if calnuc is secreted by the constitutive or regulated secretory pathway in AtT20 cells. By immunofluorescence and immunogold labeling, endogenous calnuc is found in immature secretion granules (ISG) but not mature regulated secretory granules (RSG), whereas overexpressed calnuc-green fluorescent protein (GFP) is found in both ISG and RSG, where it colocalizes with ACTH. Neither calnuc nor calnuc-GFP are released by the regulated secretory pathway, suggesting that endogenous calnuc and calnuc-GFP are progressively removed from ISG and RSG during granule maturation. We conclude that calnuc is secreted via the constitutive-like pathway and represents a useful endogenous marker for this pathway in AtT20 cells. Together, these observations indicate that calnuc has a unique itinerary as it is retained in the Golgi and is then constitutively secreted extracellularly where it may influence cell behavior via its Ca²⁺-binding properties.

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