Insights on the Quest for the Structure–Function Relationship of the Mitochondrial Pyruvate Carrier

Abstract

The molecular identity of the mitochondrial pyruvate carrier (MPC) was presented in 2012, forty years after the active transport of cytosolic pyruvate into the mitochondrial matrix was first demonstrated. An impressive amount of in vivo and in vitro studies has since revealed an unexpected interplay between one, two, or even three protein subunits defining different functional MPC assemblies in a metabolic-specific context. These have clear implications in cell homeostasis and disease, and on the development of future therapies. Despite intensive efforts by different research groups using state-of-the-art computational tools and experimental techniques, MPCs’ structure-based mechanism remains elusive. Here, we review the current state of knowledge concerning MPCs’ molecular structures by examining both earlier and recent studies and presenting novel data to identify the regulatory, structural, and core transport activities to each of the known MPC subunits. We also discuss the potential application of cryogenic electron microscopy (cryo-EM) studies of MPC reconstituted into nanodiscs of synthetic copolymers for solving human MPC2.

The Importance of Mitochondrial Pyruvate Carrier in Cancer Cell Metabolism and Tumorigenesis

“…It is worth noting, however, that Nahgampalli et al reported the functionality of human MPC2 oligomers on their own [ 86 ]. The presence of high order oligomers of MPC2 was observed during cryoelectron microscope examinations of MPC2-EGFP chimeric protein reconstituted in styrene maleic acid lipid particles, although the dominant size of the EGFP tag precluded any conclusion being drawn on the structural organization of the MPC2 oligomer [ 87 ]. Using a yeast homologous expression system, however, Tavoulari et al concluded that MPC homodimers, although they can form, are non-functional [ 81 ].…”

Structural Insights into the Human Mitochondrial Pyruvate Carrier Complexes

“…Because no crystal structures of MPC proteins are available to date, investigation of the structure–function relationship of MPC proteins remains a large challenge . Previous experimental data suggested that yeast MPC1 may have only two transmembrane helices, but MPC2 and MPC3 may contain three transmembrane helices .…”

“…Previous experimental data suggested that yeast MPC1 may have only two transmembrane helices, but MPC2 and MPC3 may contain three transmembrane helices . As a result, the yeast MPC1/2 and MPC1/3 heterodimers would use five transmembrane helices to form a functional pyruvate transport path in yeast, which seems not feasible because no other transmembrane transporters in the solute carrier (SLC) family (MPC defines a unique SLC family: SLC54A) involve only five transmembrane helices . The smallest carrier in the SLC50 family, that is, the bacterial SemiSWEET glucose transporters, whose functional unit is the homodimer of the SemiSWEET protein, has six transmembrane helices in total. , In addition, other experimental evidence suggests that the yeast MPC1 may possess three transmembrane helices such as MPC2 and MPC3 .…”

“…The smallest carrier in the SLC50 family, that is, the bacterial SemiSWEET glucose transporters, whose functional unit is the homodimer of the SemiSWEET protein, has six transmembrane helices in total. , In addition, other experimental evidence suggests that the yeast MPC1 may possess three transmembrane helices such as MPC2 and MPC3 . Because the monomeric SemiSWEET protein (∼100 residues) includes three transmembrane helices and an N-terminal amphipathic helix and thus resembles the topology proposed for yeast MPC2 and MPC3, the SemiSWEET protein has been used as a template for homology modeling of the MPC complexes. , However, because the sequence identity between the MPC and SemiSWEET protein is only ∼10%, a careful evaluation of the homology models of MPC based on the SemiSWEET protein seems necessary. Also, Phelix and co-workers have previously reported on a homology model of human and plant MPC1/2 heterodimer using RaptorX and PyDock with respiratory complex I from Escherichia coli (PDB ID: 3RKO) as the template. , Alternative template-free methods of protein modeling, de novo protein prediction, are using deep learning, such as the methods implemented in Rosetta. , These methods have been successfully applied to predict different types of protein structures including membrane proteins. − …”

Structural Biology and Structure–Function Relationships of Membrane Proteins

“…José Edwin Neciosup Quesñay, Naomi L. Pollock, Raghavendra Sashi Krishna Nagampalli, Sarah C. Lee, Vijayakumar Balakrishnan, Sandra Martha Gomes Dias, Isabel Moraes, Tim R. Dafforn and Andre Luis Berteli Ambrosio [ 12 ] provide a detailed historical review focused on the human mitochondrial pyruvate transporters (MPC1 and MPC2). Here, they tell the story of the efforts to first identify and subsequently characterise the structure and function of these integral membrane proteins.…”