Functional Expression and Characterization of a Bacterial Light-harvesting Membrane Protein inEscherichia coliand Cell-free Synthesis Systems

Heterologous expression of a bacterial light-harvesting (LH) integral membrane protein was attempted using Escherichia coli cells and cell-free synthesis systems prepared from E. coli extracts. The alpha-apoprotein of LH1 complex from purple photosynthetic bacterium Rhodospirillum rubrum was overexpressed as a recombinant protein with a histidine (His6) tag added to the carboxyl terminus. Both of the expression systems produced alpha-apoprotein in a fully functional form as can judged by its ability to form a structural subunit with native beta-apoprotein and the pigment molecule bacteriochlorophyll a. The expression product in E. coli appears to be located in the inner cell membrane and can be almost completely extracted by 0.5% (w/v) Triton X-100. Circular dichroism measurement indicated that the expressed alpha-apoproteins from both systems had alpha-helical contents essentially identical with that of the native one. About two thirds of the alpha-apoprotein expressed in E. coli was found to have the amino terminal methionine residue modified by a formyl group. About one third of the alpha-apoprotein expressed in the cell-free system was found to be oxidized at the side chain of the amino terminal methionine residue. Functional expression of the alpha-apoprotein using the cell-free system provides an useful example for producing highly hydrophobic integral membrane proteins with relatively large quantities sufficient for biophysical and structural analysis.