Cooperative transport mechanism of human monocarboxylate transporter 2

Abstract

Proton-linked monocarboxylate transporters (MCTs) must transport monocarboxylate efficiently to facilitate monocarboxylate efflux in glycolytically active cells, and transport monocarboxylate slowly or even shut down to maintain a physiological monocarboxylate concentration in glycolytically inactive cells. To discover how MCTs solve this fundamental aspect of intracellular monocarboxylate homeostasis in the context of multicellular organisms, we analyzed pyruvate transport activity of human monocarboxylate transporter 2 (MCT2). Here we show that MCT2 transport activity exhibits steep dependence on substrate concentration. This property allows MCTs to turn on almost like a switch, which is physiologically crucial to the operation of MCTs in the cellular context. We further determined the cryo-electron microscopy structure of the human MCT2, demonstrating that the concentration sensitivity of MCT2 arises from the strong inter-subunit cooperativity of the MCT2 dimer during transport. These data establish definitively a clear example of evolutionary optimization of protein function.

Basigin drives intracellular accumulation of l-lactate by harvesting protons and substrate anions

“…The prototypical, intermediate-affinity MCT1 (K m ≈ 3.5 mM) is ubiquitously expressed and mediates both, import and export of l -lactate in a physiological setting, whereas the high-affinity MCT2 (K m ≈ 0.7 mM) and the low-affinity MCT4 (K m ≈ 28 mM) appear more geared to facilitate l -lactate uptake or release, respectively [ 9 , 10 ]. The recently solved structures of a bacterial MCT homolog, and human MCT1 and MCT2 widely confirmed earlier models on the principal 12 transmembrane spanning topology and alternating access transport mechanism [ 11 – 13 ].…”

Transmembrane Facilitation of Lactate/H+ Instead of Lactic Acid Is Not a Question of Semantics but of Cell Viability

“…However, the actual proton binding site and transfer mechanisms are not known, yet [11]. Subsequently, MCTs undergo a major conformational change (Figure 7, right) that closes the cis-side and opens up the trans-side, where lactate and the proton are released [11,12].…”

“…In this study, we compared the dependence on the transmembrane pH gradient of three different types of lactate-conducting proteins. These are a strict neutral solute channel (human aquaporin 9, AQP9) [ 10 ], a prototypical secondary active, proton-driven alternating access monocarboxylate transporter (human MCT1; SLC16 family) [ 11 , 12 ], and the malaria parasite’s lactate transporter (PfFNT; microbial formate–nitrite transporter family) [ 7 , 13 ] ( Figure 1 b). The latter represents an in-between case: although the FNTs form homopentamers, their rigid protomer structure mimics the fold of an aquaporin (AQP) channel [ 14 ], whereas the lactate facilitation mechanism appears proton driven, i.e., secondary active transporter-like [ 8 , 9 ].…”

A Holistic Evolutionary and 3D Pharmacophore Modelling Study Provides Insights into the Metabolism, Function, and Substrate Selectivity of the Human Monocarboxylate Transporter 4 (hMCT4)

“…Tyr332 and Gly336, which are also involved in substrate specificity, are exposed to the channel. Compared to the recently published structure of hMCT1 in an outward-open conformation, the model exerts all known structural features with an RMSD of 0.936 Å [25]. Overall, key residues that have been identified to participate in the proton-mediated substrate translocation are exposed to the solvent, allowing for efficient pharmacophore modelling and screening for potent inhibitors of the transport channel ( Figure S4).…”