title: Protein phosphatase 2A dephosphorylates CaBP4 and regulates CaBP4 function.
abstract: OBJECTIVE
  CaBP4 is a neuronal Ca(2+)-binding protein that is expressed in the retina and in the cochlea, and is essential for normal photoreceptor synaptic function. CaBP4 is phosphorylated by protein kinase C zeta (PKCζ) in the retina at serine 37, which affects its interaction with and modulation of voltage-gated Ca(v)1 Ca(2+) channels. In this study, we investigated the potential role and functional significance of protein phosphatase 2A (PP2A) in CaBP4 dephosphorylation.
  
  
  METHODS
  The effect of protein phosphatase inhibitors, light, and overexpression of PP2A subunits on CaBP4 dephosphorylation was measured in in vitro assays. Pull-down experiments using retinal or transfected HEK293 cell lysates were used to investigate the association between CaBP4 and PP2A subunits. Electrophysiologic recordings of cotransfected HEK293 cells were performed to analyze the effect of CaBP4 dephosphorylation in modulating Ca(v)1.3 currents.
  
  
  RESULTS
  PP2A inhibitors, okadaic acid (OA), and fostriecin, but not PP1 selective inhibitors, NIPP-1, and inhibitor 2, block CaBP4 dephosphorylation in retinal lysates. Increased phosphatase activity in light-dependent conditions reverses phosphorylation of CaBP4 by PKCζ. In HEK293 cells, overexpression of PP2A enhances the rate of dephosphorylation of CaBP4. In addition, inhibition of protein phosphatase activity by OA increases CaBP4 phosphorylation and potentiates the modulatory effect of CaBP4 on Ca(v)1.3 Ca(2+) channels in HEK293T cells.
  
  
  CONCLUSIONS
  This study provides evidence that CaBP4 is dephosphorylated by PP2A in the retina. Our findings reveal a novel role for protein phosphatases in regulating CaBP4 function in the retina, which may fine tune presynaptic Ca(2+) signals at the photoreceptor synapse.
authors: Department of Physiology and Biophysics, University of Washington, Seattle, WA 98195, USA. fanfan@u.washington.edu
authors: Françoise
authors: F
authors: Haeseleer
authors: Izabela
authors: I
authors: Sokal
authors: Frederick D
authors: FD
authors: Gregory
authors: Amy
authors: A
authors: Lee
mesh_terms: D000221:Adaptation, Ocular / Q000502:physiology; D000818:Animals; D020746:Calcium Channels, L-Type / Q000502:physiology; D002135:Calcium-Binding Proteins / Q000235:genetics / Q000378:metabolism*; D003623:Dark Adaptation / Q000502:physiology; D004791:Enzyme Inhibitors / Q000494:pharmacology; D057809:HEK293 Cells; D006801:Humans; D047908:Intracellular Signaling Peptides and Proteins / Q000235:genetics / Q000378:metabolism; D051379:Mice; D008810:Mice, Inbred C57BL; D008954:Models, Biological; D009419:Nerve Tissue Proteins / Q000235:genetics / Q000378:metabolism*; D019319:Okadaic Acid / Q000494:pharmacology; D018408:Patch-Clamp Techniques; D010749:Phosphoprotein Phosphatases / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism*; D010766:Phosphorylation / Q000502:physiology; D011090:Polyenes / Q000494:pharmacology; D011493:Protein Kinase C / Q000235:genetics / Q000378:metabolism; D054648:Protein Phosphatase 2 / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism*; D000071636:Protein Phosphatase 2C; D011753:Pyrones / Q000494:pharmacology; D012160:Retina / Q000201:enzymology*; D015398:Signal Transduction / Q000502:physiology; D000099221:Protein Kinase C zeta
chemical_list: D020746:Calcium Channels, L-Type; D002135:Calcium-Binding Proteins; D004791:Enzyme Inhibitors; D047908:Intracellular Signaling Peptides and Proteins; D009419:Nerve Tissue Proteins; D019319:Okadaic Acid; D010749:Phosphoprotein Phosphatases; D011090:Polyenes; D011493:Protein Kinase C; D054648:Protein Phosphatase 2; D000071636:Protein Phosphatase 2C; D011753:Pyrones; D000099221:Protein Kinase C zeta; C494133:Cabp4 protein, mouse; C485995:Cacna1d protein, mouse; C021042:protein phosphatase inhibitor-1; C578496:PPP2CA protein, mouse; C040313:fostriecin