BTB and TAZ domain proteins (BTs) function as specialized adaptors facilitating substrate recognition of the CUL3-RING ubiquitin ligase (CRL3) complex that targets proteins for ubiquitination in reaction to diverse pressures. Nonetheless, knowledge of the molecular mechanisms by which the apple scaffold protein MdBT2 responds to external and internal signals is limited. Here, we demonstrate that a putative Ca2+ sensor, Calmodulin-like 15 (MdCML15), acts as an upstream regulator of MdBT2 to negatively modulate its functions in plasma membrane H+-ATPase regulation and iron deficiency tolerance. MdCML15 was identified to be substantially linked to MdBT2, and result in the ubiquitination and degradation of the MdBT2 target protein MdbHLH104. Consequently, MdCML15 repressed the MdbHLH104 target, MdAHA8’s expression, reducing levels of a specific membrane H+-ATPase. Finally, the phenotype of transgenic apple plantlets and calli demonstrated that MdCML15 modulates membrane H+-ATPase-produced rhizosphere pH lowering alongside iron homeostasis through an MdCML15-MdBT2-MdbHLH104-MdAHA8 pathway. Our results provide new insights into the relationship between Ca2+ signaling and iron homeostasis.

中文翻译：

---

类钙调蛋白 MdCML15 与 MdBT2 相互作用调节苹果中的铁稳态

BTB 和 TAZ 结构域蛋白 (BT) 作为专门的接头，促进 CUL3-RING 泛素连接酶 (CRL3) 复合物的底物识别，该复合物针对不同压力进行泛素化反应。尽管如此，对苹果支架蛋白 MdBT2 响应外部和内部信号的分子机制的了解仍然有限。在这里，我们证明了一种假定的 Ca2+ 传感器，钙调蛋白样 15 (MdCML15)，作为 MdBT2 的上游调节剂，负向调节其在质膜 H+-ATP 酶调节和缺铁耐受中的功能。 MdCML15 被鉴定为与 MdBT2 显着相关，并导致 MdBT2 靶蛋白 MdbHLH104 泛素化和降解。因此，MdCML15 抑制 MdbHLH104 靶标、MdAHA8 的表达，从而降低特定膜 H+-ATP 酶的水平。最后，转基因苹果植株和愈伤组织的表型表明，MdCML15 通过 MdCML15-MdBT2-MdbHLH104-MdAHA8 途径调节膜 H+-ATPase 产生的根际 pH 值降低以及铁稳态。我们的结果为 Ca2+ 信号传导与铁稳态之间的关系提供了新的见解。