Scientists have discovered how a key transport protein, called the mitochondrial ADP/ATP carrier, transports adenosine triphosphate (ATP), the chemical fuel of the cell. This process is vital to keep us alive, every second of our lives, for all of our lives. This work will help us understand how mutations can affect the function of these proteins, resulting in a range of neuromuscular, metabolic and developmental diseases.
Cellular structures, called mitochondria, are the powerhouses of our cells. Every day, we humans need our own body weight in ATP to fuel all of the cellular activities. Nerve impulses, muscle contraction, DNA replication and protein synthesis are just some examples of essential processes that depend upon a supply of ATP. Since we only have a small amount of ATP in our body, we need to remake it from the spent product ADP (adenosine diphosphate) and phosphate using an enzyme complex, called ATP synthase, which is located in mitochondria. In this way, every molecule of ATP is recycled roughly 1300 times a day. For ADP to reach the enzyme, and for the product ATP to refuel the cell, each molecule has to cross an impermeable lipid membrane that surrounds the mitochondria. The mitochondrial ADP/ATP carrier is involved in the transport of ADP in and ATP out of mitochondria.
The carrier cycles between two states; in one state, the central binding site is accessible for binding of ADP, called the cytoplasmic-open state, and in another, the binding site is accessible for binding newly synthesized ATP, called the matrix-open state. A key question has been how the protein is able to convert between these two states, changing its shape to transport ADP and ATP specifically, without letting other small molecules or ions leak across the membrane.
The paper published in Cell, describes how scientists have solved the structure of the carrier trapped in the matrix-open state. The carrier was trapped in this state by using a compound called bongkrekic acid, a lethal toxin that binds to the protein and stops it from working. The researchers could also rely on Nanobody® technology. Nanobodies are fragments of llama antibodies, which bind specifically to the matrix-open state, and the structure of carrier-nanobody complex with bound bongkrekic acid was determined by X-ray crystallography. Together with earlier structures of the cytoplasmic-open state, this discovery reveals how the carrier works at the atomic scale. The carrier is incredibly dynamic, using six moving elements to transport ADP or ATP across the membrane in a unique and carefully orchestrated way.
The cytoplasmic side of the carrier is closed by conserved hydrophobic residues, and a salt bridge network, braced by tyrosines. Glycine and small amino acid residues allow close-packing of helices on the matrix side. Uniquely, the carrier switches between states by rotation of its three domains about a fulcrum provided by the substrate-binding site.
"The development of Nanobodies against the bongkrekic acid-inhibited state was a crucial step in solving its atomic structure, as the Nanobodies facilitated the growth of crystals diffracting to high resolution and they provided an important tool to obtain the initial phases by molecular replacement," says the senior author.
The ADP/ATP carrier is just one member of a large family of related transport proteins that bring different compounds in and out of mitochondria, and based on this discovery, the scientists believe that this mechanism is likely to work in a similar way for the whole family. There are many diseases associated with dysfunction of these carriers and for the first time we understand how mutations affect their molecular function.
Structure of mitochondrial ADP/ATP transporter resolved!
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