Proximity Labeling Reveals RNA-Binding Proteins Associating with the Human Mitochondrial Import Receptor TOMM20.

The import of most mitochondrial proteins requires that their precursor proteins be bound by the peripheral receptor proteins TOM20, TOM22, and TOM70. Budding yeast TOM20 and TOM70 have been extensively studied regarding their interaction partners and recognized substrates; however, little data is available for metazoan cells. Using APEX2-based proximity labeling, we created association profiles for human TOMM20 and TOMM70 in HeLa cells. We focused particularly on their interactions with RNA-binding proteins (RBPs) because there is evidence of RNA association with the mitochondrial outer membrane (MOM) and of local translation at the mitochondrial surface, however, these processes are poorly understood. Our results demonstrate that several RBPs and translation factors preferentially associate with TOMM20 rather than TOMM70. These include SYNJ2BP, a previously identified membrane-bound RBP that binds and protects mRNA encoding mitochondrial proteins. Inhibiting translation with puromycin increased the association of these RBPs with TOMM20 compared to TOMM70. This suggests that TOMM20, but not TOMM70, may play a role in maintaining cellular homeostasis during translation stress by retaining protective RBPs and translation-related proteins at the MOM.