Volume 15, Issue 12, Pages 1277-1286 (December 2008) Directed Evolution of a Lysosomal Enzyme with Enhanced Activity at Neutral pH by Mammalian Cell-Surface Display  Kai-Chuan Chen, Chia-Hung Wu, Chuan-Yuan Chang, Wei-Cheng Lu, Qingzong Tseng, Zeljko M. Prijovich, Wolfgang Schechinger, Yen-Chywan Liaw, Yu-Lin Leu, Steve R. Roffler  Chemistry & Biology  Volume 15, Issue 12, Pages 1277-1286 (December 2008) DOI: 10.1016/j.chembiol.2008.10.008 Copyright © 2008 Elsevier Ltd Terms and Conditions

Figure 1 Characterization of Membrane-Tethered βG (A) 3T3 cells that stably expressed membrane-tethered hβG were treated with 250 (bold solid line), 125 (dashed line), 62.5 (dotted line), or 0 (regular solid line) μg/ml trypsin for 5 min at 37°C before 100 μM ELF-97 β-D-glucuronide was added to the cells at pH 6.0 for 15 min at 37°C. Surface hβG expression was detected by staining with 7G8-FITC. The cells were analyzed for FITC (surface hβG expression) or ELF-97 alcohol (hβG enzymatic activity) fluorescence on a flow cytometer. Negative control (3T3) cells are shown in gray. (B) CT26/mβG cells were stained with 25 μM ELF-97 β-D-glucuronide at pH 6.0 (solid line) or pH 7.0 (dashed line) for 5 min at room temperature. Negative control CT26 cells were stained with ELF-97 β-D-glucuronide at pH 6 (gray). (C) EJ and EJ/mβG cells were seeded separately (left and middle panels) or mixed (right panel) on cover slides before staining for βG activity (with 100 μM ELF-97 β-D-glucuronide, green) and membrane-anchored mβG expression (with biotin-labeled goat anti-HA followed by streptavidin-labeled rhodamine, red). Cells were observed under a fluorescence microscope equipped with a CCD detector. (D) CT26 and CT26/mβG cells were stained individually or in a 50/50 mixture for βG activity (25 μM ELF-97 β-D-glucuronide [pH 6.0] at room temperature for 5 min) and mβG protein expression (mAb 7G7 and goat anti-rat-FITC) and then analyzed by FACS. Chemistry & Biology 2008 15, 1277-1286DOI: (10.1016/j.chembiol.2008.10.008) Copyright © 2008 Elsevier Ltd Terms and Conditions

Figure 2 High-Throughput FACS Screening at Defined pH (A) The transgene codes for the immunoglobulin κ chain signal peptide (SP), an HA epitope (HA), hβG fused to the first extracellular domain (spacer), transmembrane domain (TM), and cytoplasmic tail (CT) of the murine B7-1 antigen. (1) Mutations were introduced in the hβG gene by error-prone PCR, DNA shuffling, or saturation mutagenesis. (2) hβG transgenes were retrovirally infected in 3T3 cells to generate a membrane-tethered hβG cell library. (3) Surface expression of hβG allowed adjustment of reaction pH for screening. The cell library was reacted with ELF-97 β-D-glucuronide to accumulate fluorescent ELF-97 alcohol in cells. (4) Cells exhibiting higher βG activity were collected and expanded for further analysis. (B) 3T3 cells expressing membrane-tethered WT (blue) or mutant (red) hβG were reacted with 25 μM ELF-97 β-D-glucuronide for 15 min at room temperature and then stained with 7G8-FITC. Cells that exhibited higher βG activity at pH 5 were isolated by FACS (left panel). After two additional rounds of sorting at pH 6.5, a population of cells with higher βG activity at pH 6.5 was isolated (right panel). (C) 3T3 fibroblasts expressing WT hβG (solid line), or unsorted (dotted line) or sorted (dashed line) collections of hβG variants, were exposed to 25 μM ELF-97 β-D-glucuronide at pH 5 (left panel) or pH 6.5 (right panel) for 15 min at room temperature. Negative control (3T3) cells are shown in gray. Chemistry & Biology 2008 15, 1277-1286DOI: (10.1016/j.chembiol.2008.10.008) Copyright © 2008 Elsevier Ltd Terms and Conditions

Figure 3 pH-Dependent Activities of Soluble hβG Variants (A) SDS-PAGE of purified WT and variant hβG proteins. Proteins were visualized by staining with Coomassie blue G-250. (B) The relative enzymatic activities (percentage of maximum activity using 0.5 mM 4-methylumbelliferyl β-D-glucuronide at 37°C for 30 min, normalized to the maximum activity of each clone) of the recombinant enzymes at the indicated pH values are shown (n = 3). Error bars, SD. (C) Purified hβG proteins were reacted with various concentrations of ELF-97 β-D-glucuronide at pH 4.5 and 7.0 to determine their kinetic constants. The kcat/Km values at pH 4.5 and 7.0 are shown (n = 3). Error bars, SD. (D) Percentage of original enzymatic activity retained by hβG proteins after incubation for 14 days at 37°C, pH 7.0. The activity was measured by the hydrolysis of 0.5 mM 4-methylumbelliferyl β-D-glucuronide at 37°C for 30 min. Error bars, SEM. Chemistry & Biology 2008 15, 1277-1286DOI: (10.1016/j.chembiol.2008.10.008) Copyright © 2008 Elsevier Ltd Terms and Conditions

Figure 4 Comparison of Prodrug Activation by Wild-Type and Mutant hβGs (A) Prodrug HAMG or SN-38G is hydrolyzed specifically by hβG to cytotoxic pHAM or SN-38. (B and C) EJ human bladder cancer cells were incubated with graded concentrations of hβG variants in RPMI (pH 6.8) containing either (B) 10 μM HAMG or (C) 100 nM SN-38G for 24 hr. The cells were further incubated for 24 hr in fresh medium before incorporation of [3H]thymidine into cellular DNA was measured. Chemistry & Biology 2008 15, 1277-1286DOI: (10.1016/j.chembiol.2008.10.008) Copyright © 2008 Elsevier Ltd Terms and Conditions

Figure 5 Mutations Involved in Enhanced Enzyme Activity at Neutral pH Values (A) Amino acid changes in the hβG variants are represented as gray spheres. (B) Docking model of ELF-97 β-D-glucuronide present in two active sites formed by interactions of two hβG monomers, colored dark and light blue, respectively. The catalytic E451 and E540 residues are represented in red. ELF-97 β-D-glucuronide and T545, E595, and T599 residues are represented with green for carbon atoms, dark blue for nitrogen atoms, red for oxygen atoms, and gray for hydrogen atoms. The figure was prepared with PyMOL (http://pymol.sourceforge.net/). Chemistry & Biology 2008 15, 1277-1286DOI: (10.1016/j.chembiol.2008.10.008) Copyright © 2008 Elsevier Ltd Terms and Conditions