Biswas Subrata, Deschênes Isabelle, Disilvestre Deborah, Tian Yanli, Halperin Victoria L, Tomaselli Gordon F
Department of Medicine, Division of Cardiology, Johns Hopkins University School of Medicine, Baltimore, MD 21205, USA.
J Gen Physiol. 2008 Mar;131(3):197-209. doi: 10.1085/jgp.200709863. Epub 2008 Feb 11.
Calmodulin (CaM) regulates steady-state inactivation of sodium currents (Na(V)1.4) in skeletal muscle. Defects in Na current inactivation are associated with pathological muscle conditions such as myotonia and paralysis. The mechanisms of CaM modulation of expression and function of the Na channel are incompletely understood. A physical association between CaM and the intact C terminus of Na(V)1.4 has not previously been demonstrated. FRET reveals channel conformation-independent association of CaM with the C terminus of Na(V)1.4 (CT-Na(V)1.4) in mammalian cells. Mutation of the Na(V)1.4 CaM-binding IQ motif (Na(V)1.4(IQ/AA)) reduces cell surface expression of Na(V)1.4 channels and eliminates CaM modulation of gating. Truncations of the CT that include the IQ region abolish Na current. Na(V)1.4 channels with one CaM fused to the CT by variable length glycine linkers exhibit CaM modulation of gating only with linker lengths that allowed CaM to reach IQ region. Thus one CaM is sufficient to modulate Na current, and CaM acts as an ancillary subunit of Na(V)1.4 channels that binds to the CT in a conformation-independent fashion, modulating the voltage dependence of inactivation and facilitating trafficking to the surface membrane.
钙调蛋白（CaM）调节骨骼肌中钠电流（Na(V)1.4）的稳态失活。钠电流失活缺陷与诸如肌强直和麻痹等病理性肌肉状况相关。CaM对钠通道表达和功能的调节机制尚未完全明确。此前尚未证实CaM与完整的Na(V)1.4 C末端之间存在物理关联。荧光共振能量转移（FRET）揭示了在哺乳动物细胞中CaM与Na(V)1.4的C末端（CT-Na(V)1.4）存在不依赖于通道构象的关联。Na(V)1.4的CaM结合IQ基序（Na(V)1.4(IQ/AA)）发生突变会降低Na(V)1.4通道的细胞表面表达，并消除CaM对门控的调节作用。包含IQ区域的CT截断会消除钠电流。通过可变长度的甘氨酸接头将一个CaM与CT融合的Na(V)1.4通道，仅在接头长度允许CaM到达IQ区域时才表现出CaM对门控的调节作用。因此，一个CaM就足以调节钠电流，并作为Na(V)1.4通道的辅助亚基，以不依赖于构象的方式与CT结合，调节失活的电压依赖性并促进向表面膜的转运。
