MPC1-like Is a Placental Mammal-specific Mitochondrial Pyruvate Carrier Subunit Expressed in Postmeiotic Male Germ Cells

Abstract

Selective transport of pyruvate across the inner mitochondrial membrane by the mitochondrial pyruvate carrier (MPC) is a fundamental step that couples cytosolic and mitochondrial metabolism. The recent molecular identification of the MPC complex has revealed two interacting subunits, MPC1 and MPC2. Although in yeast, an additional subunit, MPC3, can functionally replace MPC2, no alternative MPC subunits have been described in higher eukaryotes. Here, we report for the first time the existence of a novel MPC subunit termed MPC1-like (MPC1L), which is present uniquely in placental mammals. MPC1L shares high sequence, structural, and topological homology with MPC1. In addition, we provide several lines of evidence to show that MPC1L is functionally equivalent to MPC1: 1) when co-expressed with MPC2, it rescues pyruvate import in a MPC-deleted yeast strain; 2) in mammalian cells, it can associate with MPC2 to form a functional carrier as assessed by bioluminescence resonance energy transfer; 3) in MPC1 depleted mouse embryonic fibroblasts, MPC1L rescues the loss of pyruvate-driven respiration and stabilizes MPC2 expression; and 4) MPC1-and MPC1L-mediated pyruvate imports show similar efficiency. However, we show that MPC1L has a highly specific expression pattern and is localized almost exclusively in testis and more specifically in postmeiotic spermatids and sperm cells. This is in marked contrast to MPC1/MPC2, which are ubiquitously expressed throughout the organism. To date, the biological importance of this alternative MPC complex during spermatogenesis in placental mammals remains unknown. Nevertheless, these findings open up new avenues for investigating the structure-function relationship within the MPC complex.The metabolic balance between glycolysis and oxidative phosphorylation (OXPHOS) 2 is of crucial importance in determining cell function and fate. Rapidly proliferating cells rely preferentially on glycolysis despite its lower yield of ATP, because glycolytic intermediates efficiently meet the anabolic needs for sustained cell growth. In rapidly dividing cells, this occurs even in the presence of oxygen, a process called aerobic glycolysis, or the Warburg effect (1, 2). Differentiated cells rely more heavily on OXPHOS to meet the energy demands associated with their specialization. In addition, a shift from glycolytic to oxidative metabolism is necessary for the differentiation process to occur (2-4), and conversely, a hallmark of cancer lies in the rewiring of cellular metabolism toward increased glycolytic flux (5). Pyruvate, the end product of glycolysis, is imported into the mitochondrial matrix where it fuels the TCA cycle and thereby provides electrons and reducing equivalents to the respiratory chain. Pyruvate-derived acetyl-CoA can also be used as an anabolic substrate for the synthesis of lipids and amino acids. Thus, elucidating the molecular mechanisms that determine the intracellular fate of pyruvate is important for understanding the control of cell metabolism and ultimately of cell funct...

The yeast mitochondrial pyruvate carrier is a hetero‐dimer in its functional state

“…Specifically, in the presence of the co‐expressed MPC1, the MPC2 protein was purified alone. This is not consistent with genetic and functional analyses of mammalian MPCs, including the human MPC, where co‐expression of both MPC proteins was found to be necessary for mitochondrial pyruvate carrier activity (Bricker et al , ; Herzig et al , ; Compan et al , ; Vanderperre et al , ). We show here that the co‐expressed yeast Mpc proteins co‐purify to form folded and stable hetero‐dimers in detergent solution.…”

Multi-level analysis of reproduction in the Antarctic midge,Belgica antarctica, identifies female and male accessory gland products that are altered by larval stress and impact progeny viability

“…For example, genes involved in synthesis and transport of pyruvate, a carboxylate anion and the starting substrate of the tricarboxylic acid (TCA) cycle, could be associated with either GO term. The functional role of pyruvate metabolism in sperm viability is not known, but mitochondrial activity is demonstrated to be important for fertility, and a mitochondrial pyruvate transporter that is almost uniquely expressed in the male germline has been identified in placental mammals 98 . However, inhibition of pyruvate transporters in the mitochondrial membrane does not appear to affect sperm motility, lending support to the notion that sperm motility is fueled primarily by glycolysis (not import of pyruvate into mitochondria) 98 ;…”

“…gambiae 26 . TCA-based energy production must play a role in some other aspect of sperm viability 98 . The fact that a mitochondrial pyruvate transporter is important in sperm viability suggests that pyruvate transport and metabolism contribute to the significance of the GO terms 'tricarboxylic acid cycle,' 'monocarboxylic acid metabolic process,' and 'anion transport.'…”

“…The fact that a mitochondrial pyruvate transporter is important in sperm viability suggests that pyruvate transport and metabolism contribute to the significance of the GO terms 'tricarboxylic acid cycle,' 'monocarboxylic acid metabolic process,' and 'anion transport.' Additionally, sperm are thought to take up lactate and pyruvate through general monocarboxylic acid transporters (MCTs) in humans [98][99][100] . Perhaps there is an analogous uptake of energetically important organic acids in the seminiferous tubules of Belgica and other insects.…”

“…Perhaps there is an analogous uptake of energetically important organic acids in the seminiferous tubules of Belgica and other insects. Finally, pyruvate may be useful in supplying Acetyl-CoA for the histone acetylation that is essential for chromatin condensation during spermatogenesis [98][99][100] .…”