Inclusion bodies

Inclusion bodies are solid clumps of protein in a cell. Proteins in a cell typically stay Dissolved (Chemistry)|dissolved in the cytoplasm or cell membrane but will bind together to form solids under certain conditions. When in this form, all enzymatic activity is lost. While inclusion bodies do occur naturally, they are most often seen in recombinant protein production when attempting to produce a high concentration of protein and are often a major obstacle to scaling up production. The issue of inclusion bodies in protein expression was first described by Williams et al in 1982. The likelihood of inclusion bodies forming is dependent on many factors including the rate of translation, the protein concentration, and the nature of the protein being expressed - proteins with regions of high charge or high hydrophobicity are more likely to form inclusion bodies. The amount of a target protein that ends up in the inclusion body can be reduced by expressing the protein at a lower temperature, lower concentration of inducing agent, or by expressing as a fusion protein such as GST or especially maltose binding protein.

Proteins in inclusion bodies are typically lost as there is usually no way to make them separate and refold properly. However, in the cases where the proteins can be resolubilized into their active form, inclusion bodies can be quite helpful as a purification method- the solids are precipitated out of the cell lysate before resolubilization to yield a tremendous gain in purity. Keeping the protein insoluble during expression can also be helpful when the target protein is harmful to the host bacteria.

History
Inclusion bodies were first reported in 1982 following the attempted synthesis of proinsulin.