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PROTEIN AND VESICLE TRAFFICKING, CYTOSKELETON

Dynamic interactions between L-type voltage-sensitive calcium channel Ca_v1.2 subunits and ahnak in osteoblastic cells

Department of Biological Sciences, University of Delaware, Newark, Delaware

Submitted 14 August 2008 ; accepted in final form 3 March 2009

Voltage-sensitive Ca²⁺ channels (VSCCs) mediate Ca²⁺ permeability in osteoblasts. Association between VSCC ₁- and β-subunits targets channel complexes to the plasma membrane and modulates function. In mechanosensitive tissues, a 700-kDa ahnak protein anchors VSCCs to the actin cytoskeleton via the β₂-subunit of the L-type Ca_v1.2 (_1C) VSCC complex. Ca_v1.2 is the major ₁-subunit in osteoblasts, but the cytoskeletal complex and subunit composition are unknown. Among the four β-subtypes, the β₂-subunit and, to a lesser extent, the β₃-subunit coimmunoprecipitated with the Ca_v1.2 subunit in MC3T3-E1 preosteoblasts. Fluorescence resonance energy transfer revealed a complex between Ca_v1.2 and β₂-subunits and demonstrated their association in the plasma membrane and secretory pathway. Western blot and immunohistochemistry showed ahnak association with the channel complex in the plasma membrane via the β₂-subunit. Cytochalasin D exposure disrupted the actin cytoskeleton but did not disassemble or disrupt the function of the complex of L-type VSCC Ca_v1.2 and β₂-subunits and ahnak. Similarly, small interfering RNA knockdown of ahnak did not disrupt the actin cytoskeleton but significantly impaired Ca²⁺ influx. Collectively, we showed that Ca_v1.2 and β₂-subunits and ahnak form a stable complex in osteoblastic cells that permits Ca²⁺ signaling independently of association with the actin cytoskeleton.

osteoblasts; fluorescence resonance energy transfer; cytoskeleton

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