Whether for therapeutics, diagnostics or for characterization, some proteins just don’t cooperate when it comes to expression.
A vast number of factors may contribute to this problem. A common hurdle is that it can often be challenging for a foreign host to correctly fold a protein it does not usually produce. For instance, expression of a protein originating from a higher eukaryote is being produced in a bacterium where factors such as codon usage, translation rate, and redox potential are significantly different.
Overcoming toxicity and poor accumulation are common themes in many unsuccessful attempts to overexpress difficult proteins such as multispanning membrane proteins. In fact, a protein having multiple membrane spanning domains might not properly insert into membrane bilayers of the heterologous host or a protein might not be expressed in a soluble form.