The human MRS2 magnesium-binding domain is a regulatory feedback switch for channel activity

Mitochondrial RNA splicing 2 (MRS2) forms a magnesium (Mg²⁺) entry protein channel in mitochondria. Whereas MRS2 contains two transmembrane domains constituting a pore on the inner mitochondrial membrane, most of the protein resides within the matrix. Yet, the precise structural and functional role of this obtrusive amino terminal domain (NTD) in human MRS2 is unknown. Here, we show that the MRS2 NTD self-associates into a homodimer, contrasting the pentameric assembly of CorA, an orthologous bacterial channel. Mg²⁺ and calcium suppress lower and higher order oligomerization of MRS2 NTD, whereas cobalt has no effect on the NTD but disassembles full-length MRS2. Mutating-pinpointed residues-mediating Mg²⁺ binding to the NTD not only selectively decreases Mg²⁺-binding affinity ∼sevenfold but also abrogates Mg²⁺ binding-induced secondary, tertiary, and quaternary structure changes. Disruption of NTD Mg²⁺ binding strikingly potentiates mitochondrial Mg²⁺ uptake in WT and Mrs2 knockout cells. Our work exposes a mechanism for human MRS2 autoregulation by negative feedback from the NTD and identifies a novel gain of function mutant with broad applicability to future Mg²⁺ signaling research.