The use of automatic tools and human expertise in template-based modeling of CASP8 target proteins

Abstract

Here, we describe our template-based protein modeling approach and its performance during the eighth community-wide experiment on the Critical Assessment of Techniques for Protein Structure Prediction (CASP8, http://predictioncenter.org/casp8). In CASP8, our modeling approach was supplemented by the newly developed distant homology detection method based on sequence profile-profile comparison. Detection of structural homologs that could be used as modeling templates was largely achieved by automated profile-based searches. However, the other two major steps in template-based modeling (TBM) (selection of the best template(s) and construction of the optimal sequence-structure alignment) to a large degree relied on the combination of automatic tools and manual input. The analysis of 64 domains categorized by CASP8 assessors as TBM domains revealed that we missed correct structural templates for only four of them. The use of multiple templates or their fragments enabled us to improve over the structure of the single best PDB template in about 1/3 of our models for TBM domains. Our results for sequence-structure alignments are mixed. Although many models have optimal or near optimal sequence mapping, a large fraction contains one or more misaligned regions. Strikingly, in spite of this, our TBM models have the best overall alignment accuracy scores. This clearly suggests that the correct mapping of protein sequence onto three-dimensional structure remains one of the big challenges in protein structure prediction.