Cell membranes are the interface between an organism and its environment. These biological structures contain proteins that are extremely important for many cellular processes (e.g., nutrient uptake, metabolic waste excretion, energy metabolism, and response to external stimuli). Membrane proteins account for more than 60% of drug targets primarily because of their mediating roles between external stimuli and internal cellular metabolism.
Membrane proteins, however, are hydrophobic, which means they are insoluble and highly unstable in the aqueous environments typically used to produce and characterize soluble proteins. It is therefore difficult to produce and purify them in quantities and at a level of quality sufficient for conducting structural or functional studies. That is also why the number of unique membrane protein structures determined to date lags far behind the number of those known for soluble proteins.
There are few available systems that enable heterologous expression of membrane proteins. Most were adapted from soluble protein expression systems and usually present challenges for research projects working on membrane proteins. For example, such systems using E. coli often produce insoluble aggregates or cause host toxicity as membrane space is limited. Alternatively, eukaryotic expression systems are costly and cumbersome to implement.
In contrast, a unique system for membrane protein expression invented by Philip Laible and Deborah Hanson in the Biosciences Division at Argonne makes it possible to obtain reasonable yields of functional membrane protein. This proprietary method uses photosynthetic bacteria (Rhodobacter) for the expression of heterologous membrane proteins. Rhodobacter cells produce extremely large amounts of intracellular membrane when cultured under certain conditions. Synthesis of foreign (or native) membrane proteins and this intracellular membrane can be coordinated in these bacteria. After culture, the functional membrane protein can be purified easily by using standard protocols (e.g., affinity chromatography).
Until recently, it has been very difficult and awkward to introduce foreign DNA directly into Rhodobacter cells, as the foreign DNA is degraded in the process. Previous methods used a two-step process: foreign DNA was first introduced into E. coli cells, which were then mated with Rhodobacter cells – mobilizing and transferring the foreign DNA by a process known as conjugation. This hurdle has been removed by another Argonne invention (patent application pending): genetically engineered Rhodobacter strains that can take up foreign DNA directly using common methods (chemical or electroporation).
This method offers such advantages as lower production costs, ease of purification, scalability, and high yields (0.5 mg/L to as high as 20 mg/L). The system permits the simultaneous production and sequestration of heterologous membrane proteins, yielding a higher fraction of proteins in soluble form, as well as avoiding toxicity to the host. It has strong applications in both the pharmaceutical and biotechnology industries, and as biologics become more mainstream, large quantities of active membrane proteins will be required for regulatory testing.
Certain membrane proteins of therapeutic importance have been overexpressed with success, and research is underway to validate the method further by using other industrially relevant membrane proteins.
A platform method for membrane protein expression
Ready to transfer to industry for commercial development
J. Struct. Funct. Genomics 5: 167–172 (2004).