Overproduction of membrane proteins

All expression systems need further development to increase the expression levels and to maximise functional product and product homogeneity.

Abstract

The overproduction of membrane proteins with homologous and heterologous expression systems has proven to be more difficult than the overproduction of soluble proteins. Sugar transporters, bacteriorhodopsin and halorhodopsin can be expressed to high levels in homologous expression systems. Expression levels for functional membrane proteins in Escherichia coli are usually low. Bacteriorhodopsin and light-harvesting chlorophyll a/b binding protein have been produced in a non-native form; extraction with organic solvents or detergent solutions allowed the proteins to be reconstituted to form the active pigments. The plant plasma membrane H+-ATPase, bacteriorhodopsin, a sugar transporter and G-protein-coupled receptors have been expressed in yeast in a functional form. Transient expression and stable cell lines have been widely used to study rhodopsin and receptor mutants. A variety of membrane proteins have been expressed using the baculovirus system in insect cells in a functional form and in reasonable amounts. Several proteins have been solubilized and isolated in detergent solutions. All expression systems need further development to increase the expression levels and to maximise functional product and product homogeneity.