Efficient Screening Methodology for Protein Crystallization Based on the Counter-Diffusion Technique

We report an efficient screening methodology based on the capillary counter-diffusion technique (CCD), which was evaluated using two different practical approaches. The first consisted of kits prepared with the most successful crystallizing agents (PEG and ammonium sulfate) buffered at different pHs ranging from 4 to 9 and tested on 14 samples, including commercial and research target proteins. The second approach was based on the previously identified and highly effective 24 crystallization cocktails adapted to the counter diffusion setup. This screening was tested with two target proteins, HbII and HbII-III from the clam Lucina pectinate, and the results compared with those obtained with the vapor-diffusion experiment. The success rate was higher than 60% in both approaches. These results experimentally confirm the usefulness of the CCD technique for the screening of crystallization conditions of biomacromolecules beyond its well-known value for the growth of large and high-quality crystals. We describe a detailed protocol for the laboratory implementation of the capillary counter-diffusion technique.