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Recently advances in the identification of the tethers between the mitochondrial and ER membranes suggest that the scaffolding function of the molecular elements involved is secondary to other, non-structural functions.
In yeast, ERMES, a multiprotein complex of interacting ER-and mitochondrial-resident membrane proteins, is required for lipid transfer at the MAM and exemplifies this principle.
One of its components, for example, is also a constituent of the protein complex required for insertion of transmembrane beta-barrel proteins into the lipid bilayer.
Other proteins implicated in scaffolding likewise have functions independent of structural tethering at the MAM ; for example, ER-resident and mitochondrial-resident mitofusins form heterocomplexes that regulate the number of inter-organelle contact sites, although mitofusins were first identified for their role in fission and fusion events between individual mitochondria.
In addition to the matrix pool of grp75, a portion serves as a chaperone that physically links the mitochondrial and ER Ca < sup > 2 +</ sup > channels VDAC and IP3R for efficient Ca < sup > 2 +</ sup > transmission at the MAM.
Another prominent tether is Sigma-1R, another chaperone whose stabilization of ER-resident IP3R has been proposed to preserve communication at the MAM during the metabolic stress response.
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