Integral membrane proteins constitute a significant fraction of the proteome of all living cells. Important examples of this protein class include certain signal transduction receptors, cell adhesion molecules and ion channels. The biochemical and structural characterization of this challenging class of protein represents an important frontier in both basic science and drug discovery research. However, due to their unique physical properties and requirement for association with cellular membranes, expression of membrane proteins in heterologous systems is often daunting.
Cellular expression of recombinant membrane protein often results in protein aggregation and misfolding due to the hydrophobic nature of transmembrane segments. When using E. coli as a host, it is advantageous to express membrane proteins in moderation to avoid saturation of the membrane protein biogenesis pathway, which may lead to cell death and/or inclusion body formation. The Lemo21(DE3) protein production strain was designed for tunable T7 expression to achieve optimal assembly of transmembrane proteins or the optimal folding of soluble proteins. In standard T7 expression strains (e.g. BL21(DE3)) transcription of a target gene from a T7 promoter may be too robust and it is not easily controlled. In contrast, Lemo21(DE3) expresses a T7 RNA polymerase inhibitor protein (LysY) so that the level of target gene transcription may be precisely regulated. Lemo21(DE3) allows researchers to sample a wide range of expression levels to find the optimal conditions for each unique target protein. In the case of membrane proteins less expression often results in more functional protein.