Recombinant protein expression has become one of the major techniques to produce highly desired protein products for biotechnology, life-science and academic research.
We will talk about recombinant proteins and how they are generated by recombinant protein expression in manipulated cells. You will receive an overview of the processes involved in the biological flow of information from genetic code to the finalized protein, and you will learn about DNA and RNA without getting too technical!
Recombinant proteins are expressed from a recombinant DNA template, using molecular biology techniques.
Recombinant DNA is generated by the artificial combination of genetic material from different sources, which would not occur in nature. This allows the engineering of genes to code for artificial, optimized proteins and includes measures to facilitate higher yields.
The recombinant genes are then inserted into host cells, where they are used as blueprints for cellular protein expression.
Recombinant proteins are products of high interest in biotechnology, medicine and the life-sciences.
Examples for recombinant proteins are:
Prior to the rise of molecular biology and recombinant protein technology, these proteins had to be harvested from animal sources.
In addition, fully artificial recombinant proteins became available, such as protein fragments for vaccine production and recombinant antibodies for therapeutic and diagnostic applications.
The Encyclopedic Reference of Genomics and Proteomics in Molecular Medicine1 defines recombinant protein expression as the method of inserting a recombinantly manipulated gene into a suitable vector and its transfer into suitable cells, depending on the protein type.
Subsequently, the cells are grown in culture and then harvested to yield the recombinant protein.
In general, protein expression is the biological process of protein production in cells. The first step is the transcription of a DNA blueprint into messenger RNA. Messenger RNA is the template for a cell’s protein production machinery, the so called ribosome.
Ribosomes read the code in messenger RNAs and then assemble proteins sequentially from individual amino acids.
Recombinant protein expression uses artificial genetic material that is optimized by molecular biology. This engineering process also involves genetic signals to enhance yields. Molecular cloning methods enable scientists to combine genetic material across different species, e. g. human insulin in yeast cells.
Next, recombinant DNAs are manipulated for introduction into host cells, usually by generation of circular DNAs (plasmids) or by packaging them into viral particles. These vectors are then inserted into the host cells by processes termed transformation and transfection.
When the DNA blueprints arrive in the interior of a host cell, its own biological machinery takes over to do the remaining protein production work according to the blueprint.
First, enzymes in the cell read the DNA code and rewrite into RNA. This is the process of transcription. These enzymes read the genetic code and assemble RNA strands that contain the sequence of the desired protein.
A computer analogy is to view DNA as biological long-term data storage and RNA as short term working memory with executable instructions.
Next, ribosomes are recruited and attach to the RNA strands. They read the information encoded in the RNA and attach amino acids selectively in the right sequence to produce the full-length protein.
The finished protein is released from the ribosome, which in turn attaches to the next RNA strand. The released protein then may undergo further cellular processing steps, folding into the proper 3D shape and post-translational modifications.
Some types of host cells have proven superior for several protein production needs. Those are termed recombinant protein expression systems:
Recombinant protein expression with mammalian systems yields synthetic proteins that are made up very much like their native counterparts. The reason is the very similar biological production machinery, leading to similar post-translational modifications like glycosylation patterns.
Recombinant proteins from mammalian systems therefore show very high physiological functionality and are preferred for recombinant antibody production and protein tools for functional research.
Mammalian expression systems allow stable expression after insertion of recombinant genes into the host cell line genome.
More practically relevant is transient expression (1-2 weeks) of recombinant proteins in suspension culture. One of the most superior methods for reliable high yield of recombinant proteins is transient expression in CHO cells, for yields up to several grams per liter.