Atomic structure of human TOM core complex

The translocase of the outer mitochondrial membrane (TOM) complex is the main entry gate for mitochondrial precursor proteins synthesized on cytosolic ribosomes. Here we report the single-particle cryo-electron microscopy (cryo-EM) structure of the dimeric human TOM core complex (TOM-CC). Two Tom40 β-barrel proteins, connected by two Tom22 receptor subunits and one phospholipid, form the protein-conducting channels. The small Tom proteins Tom5, Tom6, and Tom7 surround the channel and have notable configurations. The distinct electrostatic features of the complex, including the pronounced negative interior and the positive regions at the periphery and center of the dimer on the intermembrane space (IMS) side, provide insight into the preprotein translocation mechanism. Further, two dimeric TOM complexes may associate to form tetramer in the shape of a parallelogram, offering a potential explanation into the unusual structural features of Tom subunits and a new perspective of viewing the import of mitochondrial proteins.

Glycolipozyme membrane protein integrase (MPIase): recent data

A novel factor for membrane protein integration, from the cytoplasmic membrane of Escherichia coli, named MPIase (membrane protein integrase), has recently been identified and characterized. MPIase was revealed to be essential for the membrane integration of a subset of membrane proteins, despite that such integration reactions have been, thus far, thought to occur spontaneously. The structure determination study revealed that MPIase is a novel glycolipid comprising a glycan chain with three N-acetylated amino sugars connected to diacylglycerol through a pyrophosphate linker. As MPIase catalyzes membrane protein integration, we propose that MPIase is a glycolipozyme on the basis of its enzyme-like function. The glycan chain exhibits a molecular chaperone-like function by directly interacting with substrate membrane proteins. Moreover, MPIase also affects the dimer structure of SecYEG, a translocon, thereby significantly stimulating preprotein translocation. The molecular mechanisms of MPIase functions will be outlined.

Distinct Forms of Mitochondrial TOM-TIM Supercomplexes Define Signal-Dependent States of Preprotein Sorting

ABSTRACT Mitochondrial import of cleavable preproteins occurs at translocation contact sites, where the translocase of the outer membrane (TOM) associates with the presequence translocase of the inner membrane (TIM23) in a supercomplex. Different views exist on the mechanism of how TIM23 mediates preprotein sorting to either the matrix or inner membrane. On the one hand, two TIM23 forms were proposed, a matrix transport form containing the presequence translocase-associated motor (PAM; TIM23-PAM) and a sorting form containing Tim21 (TIM23SORT). On the other hand, it was reported that TIM23 and PAM are permanently associated in a single-entity translocase. We have accumulated distinct transport intermediates of preproteins to analyze the translocases in their active, preprotein-carrying state. We identified two different forms of active TOM-TIM23 supercomplexes, TOM-TIM23SORT and TOM-TIM23-PAM. These two supercomplexes do not represent separate pathways but are in dynamic exchange during preprotein translocation and sorting. Depending on the signals of the preproteins, switches between the different forms of supercomplex and TIM23 are required for the completion of preprotein import.