Low expression and instability during isolation are major obstacles preventing adequate structure-function characterization of membrane proteins (MPs). To increase the likelihood of generating large quantities of protein, C-terminally fused green fluorescent protein (GFP) is commonly used as a reporter for monitoring expression and evaluating purification. This technique has mainly been restricted to MPs with intracellular C-termini (C in ) due to GFP's inability to fluoresce in the Escherichia coli periplasm. With the aid of Glycophorin A, a single transmembrane spanning protein, we developed a method to convert MPs with extracellular C-termini (C out ) to C in ones providing a conduit for implementing GFP reporting. We tested this method on eleven MPs with predicted C out topology resulting in high level expression. For nine of the eleven MPs, a stable, monodisperse protein-detergent complex was identified using an extended fluorescencedetection size exclusion chromatography procedure that monitors protein stability over time, a critical parameter affecting the success of structure-function studies. Five MPs were successfully cleaved from the GFP tag by site-specific proteolysis and purified to homogeneity. To address the challenge of inefficient proteolysis, we explored expression and purification conditions in the absence of the fusion tag. Contrary to previous studies, optimal expression conditions established with the fusion were not directly transferable for overexpression in the absence of the GFP tag. These studies establish a broadly applicable method for GFP screening of MPs with C out topology, yielding sufficient protein suitable for structure-function studies and are superior to expression and purification in the absence GFP fusion tagging.
Trichloroethylene (TCE) is one of the most widespread environmental contaminants, which is metabolized to N-acetyl-S-1,2-dichlorovinyl-L-cysteine (NA-DCVC) before being excreted in the urine. Alternatively, NA-DCVC can be deacetylated by aminoacylase 3 (AA3), an enzyme that is highly expressed in the kidney, liver, and brain. NA-DCVC deacetylation initiates the transformation into toxic products that ultimately causes acute renal failure. AA3 inhibition is therefore a target of interest to prevent TCE induced nephrotoxicity. Here we report the crystal structure of recombinant mouse AA3 (mAA3) in the presence of its acetate byproduct and two substrates: N α -acetyl-L-tyrosine and NA-DCVC. These structures, in conjunction with biochemical data, indicated that AA3 mediates substrate specificity through van der Waals interactions providing a dynamic interaction interface, which facilitates a diverse range of substrates.mercapturates | metalloprotein | X-ray structure A minoacylase 3 (AA3) is a member of the aminoacylase family of enzymes that deacylates a broad range of substrates including both N α -acetylated amino acids and S-cysteine conjugates of N-acetyl-L-cysteine (mercapturic acids) ( Fig. 1) (1). There are three types of aminoacylases: (i) aminoacylase 1 (AA1) deacetylates neutral aliphatic N-acyl-α-amino acids and mercapturic acids; (ii) aminoacylase 2 or aspartoacylase (AA2) has a strict specificity for N α -acetyl-L-aspartate (NAD); and (iii) aminoacylase 3 (AA3) preferentially deacetylates N α -acetylated aromatic amino acids and mercapturic acids that are usually not deacetylated by AA1 (1-6). Despite different substrate specificities, AA2 and AA3 have a high degree of sequence (42% identity) and structure homology but are both substantially different from AA1 (∼10% of sequence identity) (2, 6-10).AA3 is of particular interest for human health because it participates in mediating toxicity of the xenobiotic trichloroethylene (TCE). The United States produces in excess of 130,000 tons of TCE per year (11), making it the most widespread chemical contaminant in both soil and ground water. TCE is readily absorbed into the body where it can enter the glutathione conjugation detoxification pathway producing the mercapturic acid N-acetyl-S-1,2-dichlorovinyl-L-cysteine (NA-DCVC) for subsequent urinary excretion (12, 13). However, NA-DCVC can be deacetylated by AA3-which is highly expressed in the renal proximal tubule, liver, and brain-to generate S-1,2-dichlorovinyl-L-cysteine (DCVC) (2) and further transformed via β-lyases or flavin monooxygenases into lethal products capable of causing acute renal failure and toxicity to the liver and brain. (4, 13-22). Thus, inhibition of AA3 can decrease DCVC formation and ameliorate TCE toxicity, presenting a potential target for drug discovery.Generating specific inhibitors for AA3 would be greatly aided by high-resolution structural data, but structural studies of aminoacylases have been limited to AA1 (7) and AA2 (8, 9), which differ in both overall architecture and ac...
Crystal structures of membrane proteins are notoriously difficult to obtain. These proteins reside in phospholipid bilayers making them difficult to express, purify, stabilize and structurally resolve. Previously, inner membrane proteins (IMP) have been recombinantly expressed and monitored using a C‐terminal green fluorescent protein (GFP) fusion in Escherichia coli. This method proved extremely useful, but is only effective for IMP with C‐termini localizing to the cytoplasm (CI), where the GFP folds correctly. To fill the gap for IMP with C‐termini localizing to the periplasm (CO), we developed the expression vectors: pWarf(‐) and pWarf(+). While pWarf(‐) fuses GFP to the C‐terminus of the IMP, pWarf(+) fuses the single transmembrane helix protein Glycophorin A and subsequent GFP to the IMP. The pWarf(+) vector effectively converts CO IMP to CI IMP. Eleven E. coli IMP from three families were selected for their predicted CO topology. High level whole cell fluorescence was observed only in the pWarf(+) system. Using the pWarf(+) vector, overexpression was rapidly screened, detergents were selected by fluorescent size exclusion chromatography and stability was monitored. Large scale purification is in progress for crystallization trials. By developing the pWarf vector system, a comprehensive, streamlined approach to monitoring overexpression, purification and stabilization has been achieved for IMP. Funding from AHA.
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