Assessment of hard target modeling in CASP12 reveals an emerging role of alignment-based contact prediction methods

We present our assessment of CASP12 modeling efforts for targets with no obvious templates of high sequence/structure similarity in the PDB, that is for evaluation units of the free modeling (FM) and free modeling/template-based modeling (FM/TBM) categories. Models were clustered and ranked using the Global Distance Test-Total Score and 5 additional metrics developed in previous CASP rounds, producing short lists of models that were subject to visual inspection in comparison to the target structures. The whole procedure was implemented as a web app that facilitates model selection and visual inspection, and could become useful to facilitate and standardize future assessments. We describe cases of (1) targets with remarkably good predictions, (2) targets whose models captured some global shape and topology features, and (3) targets for which models fail to capture even coarse features. We note that despite this CASP being among the most challenging ones, a measurable improvement of the top predictions is apparent, that we attribute to the emergence of accurate contact prediction methods and the increased number of available sequences. We also briefly discuss current limitations in tertiary structure prediction exemplified by CASP12 targets. Overall, the Baker, Zhang, and Lee manual groups and servers were identified as the top global performing groups.