Intitial crystallization trials were carried out with a protein concentration of 8 mg · ml−1 (PLYD168A) or 10 mg · ml−1 (PLYΔ146/147) after addition of glycerol to a final concentration of 10% in 96-well plates by vapour diffusion. 300 nl of protein solution were mixed with 300 nl of a commercially available crystallization solution (PGA Screen from Molecular Dimensions, JB Screen Classics I from Jena Bioscience, and Classics I Suite from Qiagen) in a pipetting robot (Mosquito, TTP Labtech). Hanging drops were incubated over 100 µl of reservoir solution at 18°C. Initial crystals hits were refined by varying the protein-to-reservoir ratio with a drop volume of 3 µl, incubated over 500 µl reservoirs at 18°C in 24-well plates. PLYD168A crystals grew after one day at 18°C in a 3 µl drop of 1.5 µl protein solution (8 mg · ml-1 in 10% glycerol) and 1.5 µl of reservoir solution (0.1 M sodium cacodylate pH 6.5, 1% PGA-LM, 0.2 M potassium bromide and 0.2 M potassium thiocyanate). PLYΔ146/147 crystals grew in a few days at 18°C in a 3 µl drop of 2.0 µl protein solution (10 mg · ml-1 in 10% glycerol) and 1.0 µl of reservoir solution (0.1 M imidazole pH 6.5 and 1.2 M sodium acetate). Crystals were transferred to reservoir solution as a cryo-protectant and flash-frozen in liquid nitrogen. Data were collected at beamline PXII (Paul Scherrer Institute, Villigen, Switzerland) under a constant stream of cold nitrogen gas (100 K). Data processing, integration and scaling was performed with the XDS package (Kabsch, 1993). Structures of the PLY mutants were solved by molecular replacement with PLYWT (pdb-id 4AOD) as a search model using PHASER (McCoy, 2007) in the CCP4 software package (Collaborative Computational Project, Number 4, 1994). The initial electron density map was improved by cycles of density modification, automatic model building in RESOLVE (Terwilliger, 2004) and refinement by REFMAC (Murshudov et al., 1997). The model was subjected to iterative rounds of rebuilding into 2Fo-Fc and Fo-Fc maps using COOT (Emsley and Cowtan, 2004) and refined with the phenix.refine subroutine in the PHENIX program suite (Zwart et al., 2008). Data collection, refinement, and model statistics are summarized in Table 1. Figures were generated with PyMOL (Schrödinger, 2015).