Drosophila TRP is a calcium-permeable cation channel essential for fly visual signal transduction. During phototransduction, Ca²⁺ mediates both positive and negative feedback regulation on TRP channel activity, possibly via binding to calmodulin (CaM). However, the molecular mechanism underlying Ca²⁺ modulated CaM/TRP interaction is poorly understood. Here, we discover an unexpected, Ca²⁺-dependent binding mode between CaM and TRP. The TRP tail contains two CaM binding sites (CBS1 and CBS2) separated by an ∼70-residue linker. CBS1 binds to the CaM N-lobe and CBS2 recognizes the CaM C-lobe. Structural studies reveal the lobe-specific binding of CaM to CBS1&2. Mutations introduced in both CBS1 and CBS2 eliminated CaM binding in full-length TRP, but surprisingly had no effect on the response to light under physiological conditions, suggesting alternative mechanisms governing Ca²⁺-mediated feedback on the channel activity. Finally, we discover that TRPC4, the closest mammalian paralog of Drosophila TRP, adopts a similar CaM binding mode.

果蝇TRP 是一种钙可渗透的阳离子通道，对果蝇视觉信号转导至关重要。在光转导过程中，Ca ²⁺可能通过与钙调蛋白 (CaM) 结合来介导对 TRP 通道活性的正反馈和负反馈调节。然而，Ca ²⁺调节CaM/TRP 相互作用的分子机制知之甚少。在这里，我们发现了一个意想不到的 Ca ²⁺CaM 和 TRP 之间的依赖结合模式。TRP 尾部包含两个 CaM 结合位点（CBS1 和 CBS2），由一个~70 个残基的接头隔开。CBS1 与 CaM N 叶结合，CBS2 识别 CaM C 叶。结构研究揭示了 CaM 与 CBS1&2 的叶特异性结合。CBS1 和 CBS2 中引入的突变消除了全长 TRP 中的 CaM 结合，但令人惊讶的是对生理条件下对光的反应没有影响，这表明控制 Ca ²⁺介导的通道活性反馈的替代机制。最后，我们发现与果蝇TRP最接近的哺乳动物旁系同源物 TRPC4采用类似的 CaM 结合模式。