In spite of the fact that structure solving methods are constantly improving, the biggest challenge of protein crystallography remains the production of well diffracting single protein crystals. Full understanding the environmental factors that influence crystal packing would be an enormous task, therefore crystallographers are still forced to work "blindly" trying as many crystallizing conditions and mutations, designed to improve crystal packing, in the sequence of the target protein as possible. Numerous times the random attempts simply fail even when using crystallization robots or recent techniques to determine the optimal mutations. As an alternative option in these cases, crystallization chaperones can be used. These proteins have a unique property, namely they easily form protein crystals, which can be exploited by using them as a heterologous fusion partner to promote crystal contact formation. Today, the most frequently used crystallization chaperone is the maltose-binding protein (MBP) and crystallographers are in need of other options. Our previous results showed the outstanding crystallization properties of a non-EF hand calcium-binding protein annexin A2 (ANXA2). Here, we compared ANXA2 with the wild type MBP and found that ANXA2 is just as good, if not a better crystallization chaperone. Using ANXA2 for this purpose, we were able to solve the atomic resolution structure of a challenging crystallization target, the transactivation domain (TAD) of p53 in complex with S100A4, an EF hand calcium-binding protein associated with metastatic tumors. The full-length TAD forms an asymmetric fuzzy complex with S100A4 and could interfere with its function.

In spite of the fact that structure solving methods are constantly improving, the biggest challenge of protein crystallography remains the production of well diffracting single protein crystals. Full understanding the environmental factors that influence crystal packing would be an enormous task, therefore crystallographers are still forced to work "blindly" trying as many crystallizing conditions and mutations, designed to improve crystal packing, in the sequence of the target protein as possible. Numerous times the random attempts simply fail even when using crystallization robots or recent techniques to determine the optimal mutations. As an alternative option in these cases, crystallization chaperones can be used. These proteins have a unique property, namely they easily form protein crystals, which can be exploited by using them as a heterologous fusion partner to promote crystal contact formation. Today, the most frequently used crystallization chaperone is the maltose-binding protein (MBP) and crystallographers are in need of other options. Our previous results showed the outstanding crystallization properties of a non-EF hand calcium-binding protein annexin A2 (ANXA2). Here, we compared ANXA2 with the wild type MBP and found that ANXA2 is just as good, if not a better crystallization chaperone. Using ANXA2 for this purpose, we were able to solve the atomic resolution structure of a challenging crystallization target, the transactivation domain (TAD) of p53 in complex with S100A4, an EF hand calcium-binding protein associated with metastatic tumors. The full-length TAD forms an asymmetric fuzzy complex with S100A4 and could interfere with its function.