Magnesium transporter1 family
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The Magnesium Transporter 1 (MagT1) Family (TC# 1.A.76) is a group of magnesium transporters that are part of the TOG superfamily. Goytain and Quamme identified a Mg2+-related transporter whose expression or function was affected by an implantation-associated protein precursor.[1] They designated this protein, MagT1. MagT1 is expressed as a 335 amino acid polypeptide which includes five transmembrane helices. The nascent polypeptide possesses a cleavage site after the N-terminal signal sequence helix, rendering a mature MagT1 protein with four transmembrane helices. MagT1 additionally contains a number of phosphorylation sites.
Recent evidence demonstrates that the primary function of MagT1 is protein glycosylation, mediated by MagT1's function as a component of the oligosaccharyltransferase (OST).[2][3]
When expressed in Xenopus laevis oocytes, MagT1 mediates saturable Mg2+ uptake with a Km of 0.23 mM. Transport of Mg2+ by MagT1 is rheogenic, voltage-dependent, and does not display time-dependent inactivation. Transport is specific to Mg2+, as other divalent cations do not evoke currents. Large external concentrations of some cations inhibited Mg2+ transport (Ni2+, Zn2+, Mn2+) in MagT1-expressing oocytes although Ca2+and Fe2+ were without effect.[1] MagT1 has an N-terminal thioredoxin domain of unknown function.
Zhou and Clapham identified two mammalian genes, MagT1 and TUSC3, catalyzing Mg2+ influx.[4] MagT1 is universally expressed in all human tissues, and its expression level is upregulated in low extracellular Mg2+. Knockdown of either MagT1 or TUSC3 protein lowered the total and free intracellular Mg2+concentrations in mammalian cell lines. Morpholino knockdown of MagT1 and TUSC3 protein expression in zebrafish embryos resulted in early developmental arrest; excess Mg2+ or supplementation with mammalian mRNAs rescued these effects. Thus, MagT1 and TUSC3 are vertebrate plasma membrane Mg2+ transport system.[4]
Transport reaction
The reaction catalyzed by MagT1, or a potential downstream glycosylation target (e.g. a Mg2+ transporter), is:
- Mg2+ (out) → Mg2+ (in)