Open Access

Pamela Ornelas

• This thesis investigates the structure of the translocase of the outer membrane (TOM) complex in mitochondria, focusing on the TOM holo complex through single-particle electron cryo-microscopy (cryoEM) complemented by mass spectrometry and computational structure prediction. Mitochondria, crucial for energy production in eukaryotic cells, import most of their proteins from the cytoplasm. These proteins enter through the TOM complex, which in its core form consists of a membrane-embedded homodimer of Tom40 pores, two Tom22 cytoplasmic receptors, and six small TOM stabilizing subunits (Tom7, Tom6, and Tom5). The holo complex includes two additional subunits, Tom70 and Tom20, whose stoichiometry and positioning are less understood due to their easy dissociation during isolation of the complex. CryoEM analysis revealed the high-resolution structure of the Neurospora crassa TOM core complex at 3.3 Å, containing all core subunits, and the presence of a central phospholipid causing the Tom40 dimer to tilt to 20°. Furthermore, a 4 Å resolution map indicated the binding of a precursor protein as it transitions through the translocation barrel. Finally, at 6-7 Å resolution, the structure of the TOM holo complex highlighted Tom20's flexibility as it interacts with the core complex, emphasizing its role in protein translocation. This work provides significant insights into the architecture and functioning of the TOM complex, contributing to the understanding of mitochondrial protein import mechanisms.