Sandbox Reserved 1626: Difference between revisions

The MCU was originally thought to be composed of a [https://en.wikipedia.org/wiki/Pentamer pentamer] of five identical subunits, but it is now known to exist as a [https://en.wikipedia.org/wiki/Dimer_(chemistry) dimer] of <scene name='83/832952/Dimer_of_dimers/5'>dimers</scene>.<ref name="Woods">PMID:31869674</ref> More specifically, it is composed of two [https://en.wikipedia.org/wiki/Coiled_coil coiled-coil] domains and two transmembrane domains.<ref name="Woods"/> The hydrophobic transmembrane domain is located in the inner mitochondrial membrane ([https://en.wikipedia.org/wiki/Inner_mitochondrial_membrane IMM]) and the hydrophilic coiled-coil domain exists in the mitochondrial matrix.<ref name="Baradaran"/> The transmembrane domain (TMD) consists of eight separate helices (TM1 and TM2 from each subunit) that are connected by mostly hydrophobic amino acids in the IMS and has four-fold symmetry.<ref name="Baradaran"/>  <scene name='83/832952/Tm1/2'>TM1</scene> packs tightly against <scene name='83/832952/Tm2/2'>TM2</scene> from the neighboring subunit which conveys a sense of domain-swapping.<ref name="Fan">PMID:29995856</ref> This section of the MCU can be roughly divided into a narrow outer leaflet portion with the selectivity filter and lined by the <scene name='83/832952/Tm2/2'>TM2</scene> helices and a wide inner leaflet.<ref name="Baradaran"/> Past the transmembrane domain, the N-terminal domains of the <scene name='83/832952/Tm1/2'>TM1</scene> helices extend into the matrix and form coiled-coils with a C-terminal helix.<ref name="Baradaran"/> These "legs" are separated from each other which allows enough space for calcium ions to diffuse out into the matrix.<ref name="Baradaran"/> Additionally, this domain is responsible for assembly of the MCU and [https://en.wikipedia.org/wiki/Post-translational_modification post-translational modification].<ref name="Fan"/> Finally, each leg ends in a non-translated domain (NTD).<ref name="Baradaran"/> While the MCU can intake calcium without the NTD, it may have regulatory functions and the ability to bend transmembrane helices to constrict the pore.<ref name="Baradaran"/> <ref name="Fan"/>

There are a number of mutations that completely eliminate calcium uptake by the MCU.  For example, mutation of [https://en.wikipedia.org/wiki/Tryptophan W], [https://en.wikipedia.org/wiki/Aspartic_acid D], [https://en.wikipedia.org/wiki/Glutamic_acid E], or [https://en.wikipedia.org/wiki/Proline P] of the WDXXEP motif altered the highly conserved selectivity filter and completely eliminated calcium uptake.<ref name="Baradaran"/><ref name="Fan"/>  For example, even mutating Glu228 to an aspartate significantly changed the dimensions of the pore and inhibited uptake of calcium.<ref name="Baradaran"/> However, mutation of either X residue was not detrimental to calcium uptake.<ref name="Baradaran"/> Furthermore, mutation of a tyrosine residue directly below the selectivity filter substantially impaired calcium intake and proper protein folding.<ref name="Fan"/> The residue on TM1 that affected calcium uptake the most in human MCU was Trp317 which has a side chain constituting a primary contact point between TM1 and TM2.<ref name="Fan"/> Mutation of Phe326 or Gly331 of the TM1-TM2 linker in human MCU affected the linker conformation and configuration of the pore entrance and impaired calcium intake.<ref name="Fan"/>