Recombinant protein expression is the manufacture of a protein derived from recombinant DNA. It is a common technique in molecular biology and in pharmaceutical production of hormone replacements. The recombinant DNA is a specific portion of a gene designed to express a single product within a host cell, guided by special chemical factors so that the right protein is expressed in large amounts. Many hormones and enzymes that were historically derived from animal sources are now synthesized by recombinant protein expression, then harvested and refined from host cells.
To express recombinant proteins, carefully selected sequences of DNA must be introduced into a host's genome. Taking portions of the genetic code from one organism and placing these into the cell nuclei of another is a form of cloning. This is done via insertion of a sequence of recombinant DNA that encodes for the desired protein into the nucleus, which initiates expression of the gene by transcribing it into RNA. Recombinant proteins are assembled when pieces of mRNA carrying information from the DNA migrate to the ribosomes from the cell's nucleus, and there initiate the production of a protein according to a specific template.
Host cells will make insufficient quantities of a recombinant protein unless the DNA is introduced with appropriate vectors so that the right genetic information will be expressed in sufficient quantity. Protein expression factors are the molecular signals that must accompany recombinant DNA when it is inserted into the host cells to ensure that the target protein will be over-expressed. This is the only way recombinant protein expression can make enough of a substance for pharmaceutical or laboratory use.
Ribosomal protein assembly does not complete the protein expression process because during harvesting the bacterial or yeast cell contents get mixed with the end product. Expressed recombinant proteins must be purified by separation from the pieces of destroyed cell parts. Sometimes a molecular tag labels the protein so it can bind to a metallic or other substance and be isolated from the waste. Different techniques exist, depending on factors like protein size and the complexity of the host cell.
Human recombinant protein expression has extensive commercial and medical applications. Many hormones, antibodies, and enzymes were previously extracted from animal or cadaver tissue but are now produced synthetically using recombinant DNA technology. Two especially important examples are human growth hormone and insulin. Many hormone replacement therapies rely on synthetic proteins, as do various assays used by molecular and cellular biologists in their laboratories. In many cases, bacteria are used as host cells for simple products, while more complex recombinant protein expression, especially of genes from animals, may be done in fungi and yeasts.