right|thumb|250px|Comparison of transport proteins
Uniporters, also known as solute carriers or facilitated transporters, are a type of membrane transport protein that passively transports solutes (small molecules, ions, or other substances) across a cell membrane. It uses facilitated diffusion for the movement of solutes down their concentration gradient from an area of high concentration to an area of low concentration. Unlike active transport, it does not require energy in the form of ATP to function. Uniporters are specialized to carry one specific ion or molecule and can be categorized as either channels or carriers. Facilitated diffusion may occur through three mechanisms: uniport, symport, or antiport. The difference between each mechanism depends on the direction of transport, in which uniport is the only transport not coupled to the transport of another solute.
Uniporter carrier proteins work by binding to one molecule or substrate at a time. Uniporter channels open in response to a stimulus and allow the free flow of specific molecules. The amino acid transporters function in transporting neutral amino acids for neurotransmitter production in brain cells. This channel is activated by a voltage gradient created by sodium-potassium pumps. When the membrane reaches a certain voltage, the channels open, which depolarizes the membrane, leading to an action potential being sent down the membrane. Glucose transporters are found in the plasma membrane and play a role in transporting glucose. They help to bring glucose from the blood or extracellular space into cells usually to be utilized for metabolic processes in generating energy.
In 1855, the physiologist Adolf Fick was the first to define osmosis and simple diffusion as the tendency for solutes to move from a region of higher concentration to a lower concentration, also very well-known as Fick's Laws of Diffusion. Through the work of Charles Overton in the 1890s, the concept that the biological membrane is semipermeable became important to understanding the regulation of substances in and out of the cells. In the 1960s - 1970s, studies on the transport of glucose and other nutrients highlighted the specificity and selectivity of membrane transport proteins.
Technological advancements in biochemistry helped isolate and characterize these proteins from cell membranes. Genetic studies on bacteria and yeast identified genes responsible for encoding transporters. This led to the discovery of glucose transporters (GLUT proteins), with GLUT1 being the first to be characterized. Identification of gene families encoding various transporters, such as solute carrier (SLC) families, also advanced knowledge on uniporters and its functions.
Types
Glucose transporter (GLUTs)
The glucose transporter (GLUTs) is a type of uniporter responsible for the facilitated diffusion of glucose molecules across cell membranes.Glucose is a vital energy source for most living cells, however, due to its large size, it cannot freely move through the cell membrane. The glucose transporter is specialized in transporting glucose specifically across the membrane. The GLUT proteins have several types of isoforms, each distributed in different tissues and exhibiting different kinetic properties. Humans have been found to express fourteen GLUT proteins. Class I GLUTs include GLUT1, one of the most studied isoforms, and GLUT2.
When glucose binds to the glucose transporter, the protein channels change shape and undergo a conformational change to transport the glucose across the membrane. Once the glucose unbinds, the protein returns to its original shape. The glucose transporter is essential for carrying out physiological processes that require high energy demands in the brain, muscles, and kidneys by providing an adequate amount of energy substrate for metabolism. Diabetes, an example of a condition that involves glucose metabolism, highlights the importance of the regulation of glucose uptake in disease management.
Mitochondrial uniporter (MCU)
The mitochondrial calcium uniporter (MCU) is a protein complex located in the inner mitochondrial matrix that functions to take up calcium ions (Ca2+) into the matrix from the cytoplasm. The transport of calcium ions is specifically used in cellular function for regulating energy production in the mitochondria, cytosolic calcium signaling, and cell death. The uniporter becomes activated when cytoplasmic levels of calcium rise above 1 uM. These subunits work together to regulate the uptake of calcium in the mitochondria. Specifically, the EMRE subunit functions for the transport of calcium, and the MICU subunit functions in tightly regulating the activity of MCU to prevent the overload of calcium concentrations in the cytoplasm. LAT1 favors the transport of amino acids with large branched or aromatic side chains. The amino acid transporter functions to move essential amino acids into the intestinal epithelium, placenta, and blood-brain barrier for cellular processes such as metabolism and cell signaling. The transporter is of particular significance in the central nervous system as it provides the necessary amino acids for protein synthesis and neurotransmitter production in brain cells. Over-expression of LAT1 has been found in human cancer and is associated with playing a role in cancer metabolism.
Equilibrative nucleoside transporters (ENTs)
The nucleoside transporters, or equilibrative nucleoside transporters, are uniporters that transport nucleosides, nucleobases, and therapeutic drugs across the cell membrane. Nucleosides serve as building blocks for nucleic acid synthesis and are key components for energy metabolism in creating ATP/ GTP. They also act as ligands for purinergic receptors such as adenosine and inosine. ENTs allow the transport of nucleosides down their concentration gradient. They also have the ability to deliver nucleoside analogs to intracellular targets for the treatment of tumors and viral infections. This causes the transporter protein to change its three-dimensional shape, which ensures the substrate molecule is captured within the transporter proteins structure. The conformational change leads to the translocation of the substrate across the membrane onto the other side. The entire process depends on the substrate's concentration difference across the membrane to be the driving force for the transport by uniporters.
- Nutrient Uptake: Uniporters facilitate the transport of essential nutrients into the cell. Glucose transporters (GLUTs) are uniporters that uptake glucose for energy production.
- Metabolism: Uniporters are involved in the transport of essential ions, amino acids and molecules required for the metabolic pathway, protein synthesis and energy production
Mutations
Mutations in genes encoding uniporters lead to dysfunctional transporter proteins being formed. This loss of function in uniporters causes disruption in cellular function which leads to various diseases and disorders.
{| class="wikitable"
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!Gene with mutation
!Disease
!Result of disease
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|Mutations in the SLC2A1 gene which encode glucose transporters (GLUTs)
|GLUT1 Deficiency Syndrome
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|Mutations in the CFTR gene encoding ion channels