title: Expression, purification, and structural characterization of the bacteriorhodopsin-aspartyl transcarbamylase fusion protein.
abstract: We are testing a strategy for creating three-dimensional crystals of integral membrane proteins which involves the addition of a large soluble domain to the membrane protein to provide crystallization contacts. As a test of this strategy we designed a fusion between the membrane protein bacteriorhodopsin (BR) and the catalytic subunit of aspartyl transcarbamylase from Escherichia coli. The fusion protein (designated BRAT) was initially expressed in E. coli at 51 mg/liter of culture, to yield active aspartyl transcarbamylase and an unfolded bacterio-opsin (BO) component. In Halobacterium salinarum, BRAT was expressed at a yield of 7 mg/liter of culture and formed a high-density purple membrane. The visible absorption properties of BRAT were indistinguishable from those of BR, demonstrating that the fusion with aspartyl transcarbamylase had no effect on BR structure. Electron microscopy of BRAT membrane sheets showed that the fusion protein was trimeric and organized in a two-dimensional crystalline lattice similar to that in the BR purple membrane. Following solubilization and size-exclusion purification in sodium dodecyl sulfate, the BO portion of the fusion was quantitatively refolded in tetradecyl maltoside (TDM). Ultracentrifugation demonstrated that BR and BRAT-TDM mixed micelles had molecular masses of 138 and 162 kDa, respectively, with a stoichiometry of one protein per micelle. High TDM concentrations (>20 mM) were required to maintain BRAT solubility, hindering three-dimensional crystallization trials. We have demonstrated that BR can functionally accommodate massive C-terminal fusions and that these fusions may be expressed in quantities required for structural investigation in H. salinarum.
authors.affiliation: Department of Physiology & Biophysics, University of Miami School of Medicine, Miami, Florida, 33101, USA. gturner@chroma.med.miami.edu
authors.forename: G J
authors.initials: GJ
authors.lastname: Turner
authors.forename: L J
authors.initials: LJ
authors.lastname: Miercke
authors.forename: A K
authors.initials: AK
authors.lastname: Mitra
authors.forename: R M
authors.initials: RM
authors.lastname: Stroud
authors.forename: M C
authors.initials: MC
authors.lastname: Betlach
authors.forename: A
authors.initials: A
authors.lastname: Winter-Vann
mesh_terms: D001221:Aspartate Carbamoyltransferase / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism; D001426:Bacterial Proteins / Q000737:chemistry / Q000235:genetics; D001436:Bacteriorhodopsins / Q000737:chemistry / Q000235:genetics*; D001665:Binding Sites / Q000235:genetics; D002499:Centrifugation, Density Gradient; D003461:Crystallography; D004269:DNA, Bacterial; D004926:Escherichia coli / Q000235:genetics; D015964:Gene Expression Regulation, Bacterial; D019617:Halobacterium salinarum / Q000235:genetics; D016018:Least-Squares Analysis; D008565:Membrane Proteins / Q000737:chemistry / Q000235:genetics; D008854:Microscopy, Electron; D008958:Models, Molecular; D010957:Plasmids; D011487:Protein Conformation; D017510:Protein Folding; D011993:Recombinant Fusion Proteins / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism; D012995:Solubility; D013329:Structure-Activity Relationship; D014461:Ultracentrifugation
chemical_list: D001426:Bacterial Proteins; D004269:DNA, Bacterial; D008565:Membrane Proteins; D011993:Recombinant Fusion Proteins; D001436:Bacteriorhodopsins; D001221:Aspartate Carbamoyltransferase