The transport mechanism of the mitochondrial ADP/ATP carrier

Abstract

The mitochondrial ADP/ATP carrier imports ADP from the cytosol and exports ATP from the mitochondrial matrix, providing key steps in oxidative phosphorylation in eukaryotic organisms. The transport protein belongs to the mitochondrial carrier family, a large transporter family in the inner membrane of mitochondria. It is one of the best studied members of the family and serves as a paradigm for the molecular mechanism of mitochondrial carriers. Structurally, the carrier consists of three homologous domains, each composed of two transmembrane α--helices linked with a loop and short α--helix on the matrix side. The transporter cycles between a cytoplasmic and matrix conformation in which a central substrate binding site is alternately accessible to these compartments for binding of ADP or ATP. On both the cytoplasmic and matrix side of the carrier are networks consisting of three salt bridges each. In the cytoplasmic conformation, the matrix salt bridge network is formed and the cytoplasmic network is disrupted, opening the central substrate binding site to the intermembrane space and cytosol, whereas the converse occurs in the matrix conformation. In the transport cycle, tighter substrate binding in the intermediate states allows the interconversion of conformations by lowering the energy barrier for disruption and formation of these networks, opening and closing the carrier to either side of the membrane in an alternating way. The simultaneous 2 rotation of three domains around a central translocation pathway constitutes a unique mechanism among transport proteins.

Lipid-Loving ANTs: Molecular Simulations of Cardiolipin Interactions and the Organization of the Adenine Nucleotide Translocase in Model Mitochondrial Membranes

“…In particular, the glycine residues within these motifs were seen to form frequent contacts with CL in the simulations. The colocalization of CL interactions to these conserved sequence motifs is encouraging, given also the suggested role of the glycine residues 2 within the motifs in mediating CL interactions within crystal structures. 9 PC lipids showed comparatively less specificity for these regions (Figure S5).…”

“…69 As the number of lipid parameters and time and length scales accessible to simulations continue to increase, 54 it may be possible to address these factors in large scale simulations. With regard to mechanistic insights into the influence of CL on the ANT transport cycle, as information about the possible nature of ANT conformational states continues to emerge, 2,70 a key extension of the current simulations would be to examine the differential interactions of CL with multiple conformational states of ANT to reveal the detailed mechanism of the influence of CL on ANT function.…”

Calcium regulation of the human mitochondrial ATP-Mg/Pi carrier SLC25A24 uses a locking pin mechanism

“…The carrier exports phosphate and imports adenine nucleotides, resulting in their net uptake into mitochondria123. In contrast, the related mitochondrial ADP/ATP carrier catalyzes a strict equimolar ADP/ATP exchange, leaving the total pools of adenine nucleotides unaltered4. APC is regulated by cytosolic calcium to control the size of the matrix adenine nucleotide pool in response to cellular energetic demands123, a fundamental process for cellular growth and energy metabolism5.…”

“…It consists of three homologous sequence repeats15, each containing two membrane-spanning α-helices connected by a matrix α-helix16. Based on the identification of a conserved central substrate binding site17 flanked by two salt bridge networks on either side1618, an alternating access mechanism has been proposed for substrate transport through carrier proteins419. The regulatory domain is located in the intermembrane space, but its position relative to the carrier domain is unknown.…”

Functional Properties of the Mitochondrial Carrier System

“…Canonical mitochondrial carrier SLC25A family members harbor a tripartite structure with three tandem repeats, and functional transporters are formed by homodimerization [3, 36]. In contrast, the MPC1 and MPC2 genes encode 12 and 14 kD proteins, respectively, that associate in an unknown heteromeric stoichiometry to form the MPC.…”