Mediated transport refers to transport mediated by a

membrane transport protein A membrane transport protein (or simply transporter) is a membrane protein involved in the movement of ions, small molecules, and macromolecules, such as another protein, across a biological membrane. Transport proteins are integral transmembran ...

. Substances in the human body may be

hydrophobic In chemistry, hydrophobicity is the physical property of a molecule that is seemingly repelled from a mass of water (known as a hydrophobe). In contrast, hydrophiles are attracted to water. Hydrophobic molecules tend to be nonpolar and, th ...

electrophilic In chemistry, an electrophile is a chemical species that forms bonds with nucleophiles by accepting an electron pair. Because electrophiles accept electrons, they are Lewis acids. Most electrophiles are positively charged, have an atom that carri ...

, contain a positively or negatively charge, or have another property. As such there are times when those substances may not be able to pass over the

cell membrane The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of all cells from the outside environment ( ...

using protein-independent movement. The cell membrane is imbedded with many membrane transport proteins that allow such molecules to travel in and out of the cell. There are three types of mediated transporters: uniport,

symport A symporter is an integral membrane protein that is involved in the transport of two (or more) different molecules across the cell membrane in the same direction. The symporter works in the plasma membrane and molecules are transported across the ...

, and

antiport An antiporter (also called exchanger or counter-transporter) is a cotransporter and integral membrane protein involved in secondary active transport of two or more different molecules or ions across a phospholipid membrane such as the plasma mem ...

. Things that can be transported are nutrients, ions, glucose, etc, all depending on the needs of the cell. One example of a uniport mediated transport protein is GLUT1. GLUT1 is a transmembrane protein, which means it spans the entire width of the cell membrane, connecting the extracellular and intracellular region. It is a uniport system because it specifically transports glucose in only one direction, down its concentration gradient across the cell membrane. Another example of a uniporter mediated transport protein is

microsomal triglyceride transfer protein Microsomal triglyceride transfer protein large subunit is a protein that in humans is encoded by the ''MTTP'' gene. MTP encodes the large subunit of the heterodimeric microsomal triglyceride transfer protein. Protein disulfide isomerase (PDI) comp ...

(MTTP) who is responsible for catalyzing the assembly of the triglyceride rich lipoproteins as well mediating their release from the lumen of the

endoplasmic reticulum The endoplasmic reticulum (ER) is, in essence, the transportation system of the eukaryotic cell, and has many other important functions such as protein folding. It is a type of organelle made up of two subunits – rough endoplasmic reticulum ( ...

. What is distinguishable about this specific transfer protein is that it requires the protein PRAP1 to bind to the lipoprotein to facilitate the transport of said lipoprotein. MTTP only recognizes the PRAP1-lipoprotein complex and only then will it catalyze the transport reaction. In a way, the PRAP1 protein acts as a signal for MTTP. The importance of such interactions implies that mediated transport is not only dependent on transmembrane proteins but can also require the presence of additional non-transmembrane proteins. For instance, studies show that in the absence of a fully functional PRAP1 protein, MTTP fails to transport specific lipoproteins across the endoplasmic reticulum membrane. An example of a symporter mediated transport protein is SGLT1, a sodium/glucose co-transporter protein that is mainly found in the intestinal tract. The SGLT1 protein is a symporter system because it passes both glucose and sodium in the same direction, from the lumen of the intestine to inside the intestinal cells. An example of an antiporter mediated transport protein is the sodium-calcium antiporter, a transport protein involved in keeping the cytoplasmic concentration of calcium ions in the cells, low. This transport protein is an antiporter system because it transports three sodium ions across the plasma membrane in exchange for a calcium ion, which is transported in the opposite direction. TransportProteine

Mechanism of transport. A molecule will bind to a transporter protein, altering its shape. The change of shape or other added substances such as ATP will, in turn, cause the transport protein to alter its shape and release the molecule onto the other side of the cell membrane.

Types of Transport

Facilitated diffusion Facilitated diffusion (also known as facilitated transport or passive-mediated transport) is the process of spontaneous passive transport (as opposed to active transport) of molecules or ions across a biological membrane via specific transmembra ...

Active transport In cellular biology, ''active transport'' is the movement of molecules or ions across a cell membrane from a region of lower concentration to a region of higher concentration—against the concentration gradient. Active transport requires cellul ...

Mutations in Transport Proteins

The importance of mediated transport proteins is visualized with the presence of

mutations In biology, a mutation is an alteration in the nucleic acid sequence of the genome of an organism, virus, or extrachromosomal DNA. Viral genomes contain either DNA or RNA. Mutations result from errors during DNA or viral replication, mi ...

that render the transport proteins nonfunctional. A prime example of this are mutations found within the

Archain 1 Archain 1 is a protein that in humans is encoded by the ARCN1 gene. Function This gene maps in a region, which include the mixed lineage leukemia and Friend leukemia virus integration 1 genes, where multiple disease-associated chromosome tra ...

gene which codes for the transport proteins

COPI COPI is a coatomer, a protein complex that coats vesicles transporting proteins from the ''cis'' end of the Golgi complex The Golgi apparatus (), also known as the Golgi complex, Golgi body, or simply the Golgi, is an organelle found in most ...

and

COPII The Coat Protein Complex II, or COPII, is a group of proteins that facilitate the formation of vesicles to transport proteins from the endoplasmic reticulum to the Golgi apparatus or endoplasmic-reticulum–Golgi intermediate compartment. This ...

. The main function of these transport proteins is to facilitate the passage of molecules from the endoplasmic reticulum to the

golgi apparatus The Golgi apparatus (), also known as the Golgi complex, Golgi body, or simply the Golgi, is an organelle found in most eukaryotic cells. Part of the endomembrane system in the cytoplasm, it packages proteins into membrane-bound vesicles ins ...

, and vice versa. The mutated ARCN1 gene gives rise to abnormal COPI who fails to transport type I collagen and leads to the secretion of collagen. Due to the fact that

type I collagen Type I collagen is the most abundant collagen of the human body. It forms large, eosinophilic fibers known as collagen fibers. It is present in scar tissue, the end product when tissue heals by repair, as well as tendons, ligaments, the endomys ...

is the main ingredient of

connective tissue Connective tissue is one of the four primary types of animal tissue, along with epithelial tissue, muscle tissue, and nervous tissue. It develops from the mesenchyme derived from the mesoderm the middle embryonic germ layer. Connective tiss ...

, such mutations are the cause of numerous severe skeletal disorders such as

osteogenesis imperfecta Osteogenesis imperfecta (; OI), colloquially known as brittle bone disease, is a group of genetic disorders that all result in bones that break easily. The range of symptoms—on the skeleton as well as on the body's other organs—may be mi ...

and

cranio-lenticulo-sutural dysplasia Cranio-lenticulo-sutural dysplasia (CLSD, or Boyadjiev–Jabs syndrome) is a neonatal/infancy disease caused by a disorder in the 14th chromosome. It is an autosomal recessive disorder, meaning that both recessive genes must be inherited from eac ...

. Various variations of these disorders are characterized by visible physical dysplasia. This example highlights the importance of transport proteins, not only as a means for the passage of specific molecules across a membrane, but for proper bodily development.

References

External links

*Voet, Donald; Voet, Judith G.; Pratt, Charlotte W
''Fundamentals of Biochemistry: Life at the Molecular Level''

Chapter 10. "Membrane Transport"
p. 286-310 Transport phenomena Transport proteins {{Biochem-stub