Tail-anchored (TA) proteins are anchored to their corresponding membrane via a single transmembrane segment (TMS) at their C-terminus. In yeast, the targeting of TA proteins to the endoplasmic reticulum (ER) can be mediated by the guided entry of TA proteins (GET) pathway, whereas it is not yet clear how mitochondrial TA proteins are targeted to their destination. It has been widely observed that some mitochondrial outer membrane (MOM) proteins are mistargeted to the ER when overexpressed or when their targeting signal is masked. However, the mechanism of this erroneous sorting is currently unknown. In this study, we demonstrate the involvement of the GET machinery in the mistargeting of suboptimal MOM proteins to the ER. These findings suggest that the GET machinery can, in principle, recognize and guide mitochondrial and non-canonical TA proteins. Hence, under normal conditions, an active mitochondrial targeting pathway must exist that dominates the kinetic competition against other pathways.

Mitochondria are separated from the remainder of the eukaryotic cell by the mitochondrial outer membrane (MOM). The MOM plays an important role in different transport processes like lipid trafficking and protein import. In yeast, the ER-mitochondria encounter structure (ERMES) has a central, but poorly defined role in both activities. To understand the functions of the ERMES, we searched for suppressors of the deficiency of one of its components, Mdm10, and identified a novel mitochondrial protein that we named Mdm10 complementing protein 3 (Mcp3). Mcp3 partially rescues a variety of ERMES-related phenotypes.We further demonstrate that Mcp3 is an integral protein of the MOM that follows a unique import pathway. It is recognized initially by the import receptor Tom70 and then crosses the MOM via the translocase of the outer membrane. Mcp3 is next relayed to the TIM23 translocase at the inner membrane, gets processed by the inner membrane peptidase (IMP) and finally integrates into the MOM. Hence, Mcp3 follows a novel biogenesis route where a MOM protein is processed by a peptidase of the inner membrane.

Regulation of the localization of mRNAs and local translation are universal features in eukaryotes and contribute to cellular asymmetry and differentiation. In Saccharomyces cerevisiae, localization of mRNAs that encode membrane proteins requires the She protein machinery, including the RNA-binding protein She2p, as well as movement of the cortical endoplasmic reticulum (cER) to the yeast bud. In a screen for ER-specific proteins necessary for the directional transport of WSC2 and EAR1 mRNAs, we have identified enzymes that are involved in phospholipid metabolism. Loss of the phospholipid methyltransferase Cho2p, which showed the strongest impact on mRNA localization, disturbs mRNA localization, as well as ER morphology and segregation, owing to an increase in the amount of cellular phosphatidylethanolamine (PtdEtn). Mislocalized mRNPs containing She2p colocalize with aggregated cER structures, suggestive of the entrapment of mRNA and She2p by the elevated PtdEtn level. This was confirmed by the elevated binding of She2p to PtdEtn-containing liposomes. These findings underscore the importance of ER membrane integrity in mRNA transport.

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Mitochondria are organelles containing two membranes that are distinct in composition and function. A role of the mitochondrial outer membrane (MOM) is to mediate contact of the organelle with the rest of the cell. In yeast, the MOM contains about 40 different integral proteins. Recently, we described the MOM protein Mcp3, which can serve as a multicopy suppressor of loss of ERMES complex that mediates mitochondria-endoplasmic reticulum contacts. To shed further light on the role of Mcp3 in the MOM, we analyzed its physical interaction with other proteins. We show that Mcp3 interacts with the MOM protein Om45 and the inner membrane protein Aim19. Our observations hint toward a potential involvement of Mcp3 in a structural and/or functional link between both mitochondrial membranes.

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