The calcyclin-binding protein (CacyBP) binds calcyclin (S100A6) at physiological levels of [Ca(2+)] and is highly expressed in brain neurons. Subcellular localization of CacyBP was examined in neurons and neuroblastoma NB-2a cells at different [Ca(2+)](i). Immunostaining indicates that CacyBP is present in the cytoplasm of unstimulated cultured neurons in which resting [Ca(2+)](i) is known to be approximately 50 nm. When [Ca(2+)](i) was increased to above 300 nm by KCl treatment, the immunostaining was mainly apparent as a ring around the nucleus. Such perinuclear localization of CacyBP was observed in untreated neuroblastoma NB-2a cells in which [Ca(2+)](i) is approximately 120 nm. An additional increase in [Ca(2+)](i) to above 300 nm by thapsigargin treatment did not change CacyBP localization. However, when [Ca(2+)](i) in NB-2a cells dropped to 70 nm, because of BAPTA/AM treatment, perinuclear localization was diminished. Ca(2+)-induced translocation of CacyBP was confirmed by immunogold electron microscopy and by fluorescence of NB-2a cells transfected with an EGFP-CacyBP vector. Recombinant CacyBP can be phosphorylated by protein kinase C in vitro. In untreated neuroblastoma NB-2a cells, CacyBP is phosphorylated on a serine residue(s), but exists in the dephosphorylated form in BAPTA/AM-treated cells. Thus, phosphorylation of CacyBP occurs in the same [Ca(2+)](i) range that leads to its perinuclear translocation.
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