Vishnu, N. | Search bioRxiv

bioRxiv2022

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

Mitochondrial RNA splicing protein 2 (MRS2) forms a major magnesium (Mg2+) entry channel into the matrix. While MRS2 contains two transmembrane domains that constitute a pore, 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 function 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, while 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 [~]7-fold but also abrogates Mg2+ binding-induced changes in secondary, tertiary and quaternary structures. Disruption of NTD Mg2+ binding strikingly potentiates mitochondrial Mg2+ uptake in wild-type 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.

biophysics↗

bioRxiv2019

Mitochondrial clearance of Ca2+ controls insulin secretion

Transport of Ca2+ from the cytosol to the mitochondrial matrix of insulin-secreting pancreatic {beta}-cells facilitates nutrient-mediated insulin secretion. However, the underlying mechanism is unclear. The establishment of the molecular identity of the mitochondrial Ca2+ uniporter (MCU) and associated proteins has allowed mitochondrial Ca2+ transport to be modified in intact cells. We examined the consequences of deficiency of the accessory protein, MICU2, in rat and human insulin-secreting cell lines as well as in mouse islets. Glucose-induced mitochondrial Ca2+ elevation and inner membrane hyperpolarization were reduced, together with cytosolic ATP/ADP-ratios and insulin secretion. Insulin secretion in Micu2 knock out mice was attenuated in vitro as well as in vivo. While KCl-evoked sub-plasmalemmal Ca2+ increases were more pronounced, the global cytosolic Ca2+ response was, surprisingly, diminished in MICU2-deficient cells. These findings were supported by selective inhibition of mitochondrial Ca2+ uptake by mitochondrial depolarization. It is concluded that mitochondrial Ca2+ transport plays an additional and hitherto unrecognized role in stimulated {beta}-cells by regulating net Ca2+ entry across the plasma membrane. This is likely accounted for by clearing of sub-plasmalemmal Ca2+ levels by mitochondria located near the plasma membrane.

molecular biology↗