Roles of additives and precipitants in crystallization of calcium- and integrin-binding protein

The interactions leading to crystallization of calcium- and integrin-binding protein (CIB), a novel Ca2+' binding protein, were characterized in terms of osmotic second virial coefficients, B-22, measured by self-interaction chromatography. In particular, the role of additives such as alkanediols or DMSO in improving crystal growth in the presence of sodium malonate, sodium formate, and sodium acetate was investigated. Short-chain alkane-1 n-diols were found to be effective in a range of 10-20% (v/v) in preventing CIB precipitation at the high sodium formate concentrations necessary for crystallization, whereas for alkane-1,2-diols longer chains were necessary, although at concentrations less than 5% (v/v). In both cases, significant improvement in protein crystal growth and suppression of precipitation were observed. Isothermal titration calorimetry measurements indicate that the differences in alkanediol specificity are a result of interactions with hydrophobic regions on the protein surface, which in turn depend on chain length and structure. Overall, our investigation of the protein-protein interactions provides valuable insight into the modulating roles that additives play, which is essential to developing rational approaches to protein crystallization. Our work also demonstrates the applicability of self-interaction chromatography to studying crystallization of novel proteins, particularly those that may otherwise be difficult to crystallize due to poor solubility.