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The Journal of Neuroscience, November 14, 2007, 27(46):12743-12754; doi:10.1523/JNEUROSCI.4264-07.2007

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Cellular/Molecular
Phosphorylation of the Ca²⁺-Binding Protein CaBP4 by Protein Kinase C in Photoreceptors

Amy Lee,¹ Amber Jimenez,² Guiying Cui,¹ and Françoise Haeseleer²

¹Department of Pharmacology and Center for Neurodegenerative Disease, Emory University School of Medicine, Atlanta, Georgia 30322, and ²Department of Ophthalmology, University of Washington, Seattle, Washington 98195

Correspondence should be addressed to Françoise Haeseleer, University of Washington, Department of Ophthalmology, 1959 North East Pacific Street, Box 356485, Seattle, WA 98195. Email: fanfan{at}u.washington.edu

CaBP4 is a calmodulin-like neuronal calcium-binding protein that is crucial for the development and/or maintenance of the cone and rod photoreceptor synapse. Previously, we showed that CaBP4 directly regulates Ca_v1 L-type Ca²⁺ channels, which are essential for normal photoreceptor synaptic transmission. Here, we show that the function of CaBP4 is regulated by phosphorylation. CaBP4 is phosphorylated by protein kinase C (PKC) at serine 37 both in vitro and in the retina and colocalizes with PKC in photoreceptors. CaBP4 phosphorylation is greater in light-adapted than dark-adapted mouse retinas. In electrophysiological recordings of cells transfected with Ca_v1.3 and CaBP4, mutation of the serine 37 to alanine abolished the effect of CaBP4 in prolonging the Ca²⁺ current through Ca_v1.3 channel, whereas inactivating mutations in the CaBP4 Ca²⁺-binding sites strengthened Ca_v1.3 modulation. These findings demonstrate how light-stimulated changes in CaBP4 phosphorylation and Ca²⁺ binding may regulate presynaptic Ca²⁺ signals in photoreceptors.

Key words: CaBP4; Ca²⁺-binding proteins; phosphorylation; protein kinase C ; Ca²⁺ channel; photoreceptors

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Received June 22, 2007; accepted Oct. 9, 2007.

Correspondence should be addressed to Françoise Haeseleer, University of Washington, Department of Ophthalmology, 1959 North East Pacific Street, Box 356485, Seattle, WA 98195. Email: fanfan{at}u.washington.edu

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