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.
