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Fusion proteins, AKA chimeric proteins, are proteins created through the joining of two or more genes which originally coded for separate proteins. Translation of this fusion gene results in a single polypeptide with functional properties derived from each of the original proteins. Recombinant fusion proteins are created artificially by recombinant DNA technology for use in biological research or therapeutics. Chimeric mutant proteins occur naturally when a large-scale mutation, typically a chromosomal translocation, creates a novel coding sequence containing parts of the coding sequences from two different genes. Naturally occurring fusion proteins are important in cancer, where they may function as oncoproteins. The bcr-abl fusion protein is a well-known example of an oncogenic fusion protein, and is considered to be the primary oncogenic driver of chronic myelogenous leukemia.

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Properties of fusion proteins

The functionality of fusion proteins is made possible by the fact that many protein functional domains are modular. In other words, the linear portion of a polypeptide which corresponds to a given domain, such as a tyrosine kinase domain, may be removed from the rest of the protein without destroying its intrinsic enzymatic capability.

Recombinant fusion proteins

A recombinant fusion protein is a protein created through genetic engineering of a fusion gene. This typically involves removing the stop codon from a cDNA sequence coding for the first protein, then appending the cDNA sequence of the second protein in frame through ligation or overlap extension PCR. That DNA sequence will then be expressed by a cell as a single protein. The protein can be engineered to include the full sequence of both original proteins, or only a portion of either.

If the two entities are proteins, often linker (or "spacer") peptides are also added which make it more likely that the proteins fold independently and behave as expected. Especially in the case where the linkers enable protein purification, linkers in protein or peptide fusions are sometimes engineered with cleavage sites for proteases or chemical agents which enable the liberation of the two separate proteins. This technique is often used for identification and purification of proteins, by fusing a GST protein, FLAG peptide, or a hexa-his peptide (aka: a 6xhis-tag) which can be isolated using nickel or cobalt resins (affinity chromatography). Chimeric proteins can also be manufactured with toxins or anti-bodies attached to them in order to study disease development.

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Chimeric protein drugs

The purpose of creating fusion proteins in drug development is to impart properties from each of the "parent" proteins to the resulting chimeric protein. Several chimeric protein drugs are currently available for medical use.

Most chimeric protein drugs are "humanized" monoclonal antibodies whose specificity for a target molecule was developed using mice and hence were initially "mouse" antibodies. As non-human proteins, mouse antibodies tend to evoke an immune reaction if administered to humans. The "humanization" process involves engineering the replacement of segments of the antibody molecule that distinguish it from a human antibody. For example, a human Fc portion could be introduced, thereby eliminating most of the potentially immunogenic portions of the drug without altering its specificity for the intended therapuetic target. Antibody nomenclature indicates chimeric nature by the -xi- section of the non-proprietary name (e.g. Abci-xi-mab. Similarly, humanized antibodies (although not conceptually distinct from chimeras) are indicated using -zu- such as in Dacli-zu-mab. See the list of monoclonal antibodies for more examples.

In addition to humanized antibodies there are also other pharmaceutical purposes for the creation of chimeric constructs. Etanercept, for example, is a TNF╬▒ blockers created through the combination of tumor necrosis factor receptor (TNFR) and the IgG1 Fc segment (where TNFR provides specificity for the drug target and the antibody Fc segment is believed to add stability and deliverablity of the drug).

Naturally occurring fusion proteins

Naturally occurring fusion genes are most commonly created when a chromosomal translocation replaces the terminal exons of one gene with intact exons from a second gene. This creates a single gene which can be transcribed, spliced, and translated to produce a functional fusion protein. Many important cancer-promoting oncogenes are fusion genes produced in this way.

Examples include:

Antibodies are fusion proteins produced by VDJ recombination.

See also

External links


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