The accuracy of protein structures in solution determined by AlphaFold and NMR

In the recent CASP (Critical Assessment of Structure Prediction) competition, AlphaFold2 performed outstandingly. Its worst predictions were for NMR structures, which has two alternative explanations: either the NMR structures were poor, implying that AlphaFold may be more accurate than NMR; or there is a genuine difference between crystal and solution structures. Here, we use the program ANSURR, which measures the accuracy of solution structures, and show that one of the NMR structures was indeed poor. We then compare AlphaFold predictions to NMR structures, and show that AlphaFold tends to be more accurate than NMR ensembles, in particular correctly more rigid in loops. There are however some cases where the NMR ensembles are more accurate. These tend to be dynamic structures where AlphaFold had low confidence. We suggest that AlphaFold could be used as the model for NMR structure refinements, and that AlphaFold structures validated by ANSURR require no further refinement.