![]() Surprisingly, the fission and fusion machineries co-localize at the same ER-mitochondria MCSs, not separate ones ( Abrisch et al., 2020). MCSs between the ER and mitochondria define the position where mitochondria undergo fission and fusion ( Abrisch et al., 2020 Friedman et al., 2011 Guo et al., 2018). ABHD16A is not only required for ER-associated mitochondrial constriction, but it is also the first ER protein to be shown to be required for both fission and fusion machinery to assemble at contact sites. Using this strategy, we have identified an ER membrane-localized lipid hydrolase, ABHD16A, that could alter lipid membranes for mitochondrial constriction after ER contact sites are established. We have fused TurboID to the known fusion machinery, Mfn1, to biotinylate and subsequently identify neighboring ER proteins that could regulate these nodes. ![]() To identify an ER machinery involved in node formation, we have taken advantage of a promiscuous biotin ligase (TurboID) that can biotinylate proteins within a ~10–30 nm range upon biotin addition ( Branon et al., 2018 Roux et al., 2012). We hypothesized that an ER membrane protein facilitates the formation of these nodes. However, it is not known how these nodes are formed or regulated. These predefined branch points, or nodes, are where both fission (Drp1) and fusion (Mfn1) machineries converge ( Abrisch et al., 2020). ![]() We have recently discovered that cycles of fission and fusion occur at the same location or hot spots that are spatially dictated by the ER. However, how the ER contributes to defining a site on mitochondria that is primed for constriction and sufficient to coordinate the recruitment of both fission and fusion machineries and further how these two seemingly opposing activities are co-recruited and also balanced is unclear. Subsequently, inner mitochondrial membrane (IMM) fusion occurs via the GTPase, Opa1 ( Ban et al., 2017 Herlan et al., 2003 Lee et al., 2004 Legros et al., 2002 Meeusen et al., 2006 Misaka et al., 2002). ER-mitochondria MCSs also dictate sites where OMM fusion occurs via Mfn1/2 oligomerization in trans to drive membrane fusion upon GTP hydrolysis ( Abrisch et al., 2020 Chen et al., 2003 Guo et al., 2018 Santel and Fuller, 2001). Subsequently, two GTPases (Drp1 and Dyn2) are recruited to the OMM at ER MCSs to further constrict the OMM, which leads to mitochondrial division ( Bleazard et al., 1999 Ferguson and De Camilli, 2012 Labrousse et al., 1999 Lee et al., 2016 Smirnova et al., 1998). Several factors have been linked to ER-associated mitochondrial fission, including two actin nucleators, INF2 and Spire1c, which are proposed to polymerize actin at ER-mitochondria MCSs to initiate constriction of the outer mitochondrial membrane (OMM Korobova et al., 2013 Manor et al., 2015). Both processes are first initiated by the endoplasmic reticulum (ER) at ER-mitochondria membrane contact sites (MCSs). Disruption of these cycles results in fragmentation or elongation, which can be detrimental to cell health and is associated with various disease states ( Rambold et al., 2011 Wai and Langer, 2016). Mitochondria maintain their overall architecture and morphology by undergoing cycles of fission and fusion ( Friedman et al., 2010 Twig et al., 2008 Youle and van der Bliek, 2012). Editor's evaluationĬells maintain a characteristic mitochondrial architecture important for cellular metabolism and function. Our data present the first example of an ER membrane protein that regulates the recruitment of both fission and fusion machineries to mitochondria. These data suggest a mechanism whereby ABHD16A functions by altering phospholipid composition at ER-mitochondria MCSs. ABHD16A contains an acyltransferase motif and an α/β hydrolase domain, and point mutations in critical residues of these regions fail to rescue the formation of ER-associated mitochondrial hot spots. In the absence of ABHD16A, fission and fusion machineries fail to recruit to ER-associated mitochondrial nodes, and fission and fusion rates are significantly reduced. We have used a promiscuous biotin ligase linked to the fusion machinery, Mfn1, and proteomics to identify an ER membrane protein, ABHD16A, as a major regulator of node formation. We set out to identify how ER-associated mitochondrial nodes can regulate both fission and fusion machinery assembly. These MCSs or nodes co-localize fission and fusion machinery. Mitochondria are dynamic organelles that undergo cycles of fission and fusion at a unified platform defined by endoplasmic reticulum (ER)-mitochondria membrane contact sites (MCSs).
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