The term antigen is used to describe any substance which induces an immune response. Foreign antigens include bacteria, viruses, fungi, toxins, and any other material that originates outside the body. An immune response may also be raised against self-antigens, as exemplified by the destruction of insulin-producing pancreatic beta cells during type I diabetes.
Antigens are essential to the production of antibodies for research. Following immunisation of a suitable host with the antigen of interest, an immune response results in antibody secretion by B-cells. Polyclonal antibody preparations are produced by purifying the secreted antibodies from the host’s serum via methods such as Protein A/G purification or antigen-specific affinity purification. Monoclonal antibody production is more involved, requiring fusion of the immunised host’s spleen cells with immortal myeloma cells, before isolation and expansion of antibody-producing clones.
When selecting an antibody for a specific research application, it is important that researchers consider the nature of the antigen used to produce it. For example, if the intention is to use the antibody to detect a truncated protein which has been used as a positive control on a Western blot, and that protein is derived from the N-terminus of the full-length biomolecule, an antibody which was raised against the C-terminus is unlikely to achieve the desired immunostaining result. Antigen information is usually provided on antibody datasheets.
A peptide is defined as being a short chain of two or more amino acids, covalently attached to one another by peptide bonds. There is no set length to distinguish a peptide from a protein, however peptides are considerably shorter than proteins and have a far less complex, typically linear, structure.
Peptides occur naturally in the body, where they perform many essential functions. Examples of peptide hormones include insulin, glucagon and glucagon-like peptide 1 (GLP-1), which are involved in regulating blood glucose; ghrelin and peptide tyrosine tyrosine (PYY), which are important to control appetite; and bradykinin, which plays a key role in governing blood pressure. Other naturally-occurring peptides include the antimicrobial bacteriocins and microcins; and the lytic peptide melittin, a major component of bee venom which has been studied for its potential utility as an anti-cancer agent.
Peptides usually exert a downstream function by binding to specific receptors. This results in activation of signaling cascades to produce a wide variety of effects. For example, peptide binding to a G-protein-coupled receptor (GPCR) may cause the GPCR to interact with other membrane proteins involved in signal transduction to target ion channels in the cell membrane.
Synthetic peptides are widely-used as immunogens during antibody production. Since they are often too small to elicit a substantial immune response, they are usually conjugated to a large carrier protein such as ovalbumin, bovine serum albumin (BSA) or keyhole limpet haemocyanin (KLH). Peptide immunogens often contain modifications such as phosphorylation, acetylation, glycosylation or methylation, allowing the production of antibodies suitable to study complex processes like protein activation or silencing.
To meet demand for supply, proteins are often produced recombinantly using bacterial systems, insect cells, or mammalian cell lines such as Chinese hamster ovary (CHO) cells or human embryonic kidney (HEK293) cells. This process involves cloning the gene of interest into a suitable vector, which is subsequently introduced to the host cells. The cell line is then expanded, and the protein purified. With recent advances in cloning and purification technologies, recombinant protein expression has become an established technique.
In addition to improved yields, a further advantage of recombinant technology is that it provides the capacity to engineer desirable features into the protein. Tags may be introduced to facilitate protein purification or detection, or the protein may be modified in some way to improve solubility or bioactivity. Recombinant technology also affords the production of specific mutants, enabling an enhanced understanding of certain disease states. It can also be used to rapidly generate large numbers of proteins for immobilisation on protein-based microarrays.
When choosing a recombinant protein for a research application, researchers should consider the exact nature of the protein to ensure sensible interpretation of results. For example, if a recombinant protein is intended for use as a positive control on a Western blot, it is important to note that a truncated protein will produce a detectable band which is smaller than the theoretical molecular weight.
Recombinant proteins are often engineered to contain a specific peptide or protein sequence, known as a tag. Affinity tags are exploited during protein purification, while tags can also be used to enhance protein stability or solubility. Epitope tags allow for protein detection, for example using anti-tag antibodies, whereas fluorescent tags aid visualisation.
Commonly-used tags include glutathione S-transferase (GST), maltose-binding protein (MBP), calmodulin-binding peptide (CBP), polyhistidine (His), Myc, FLAG, HA, V5, SUMO and a wide variety of fluorescent dyes and proteins. Tags vary considerably in size, a factor which is especially of note if a tagged protein is intended for use as a Western blotting control.
Tags are typically added to the N- or C-terminus of the encoded protein, since this approach is less likely to impact on protein folding, however it is also possible to place tags elsewhere within the protein structure. Many affinity tags include a recognition sequence to allow their removal following protein purification.