The successful prediction of protein structure from amino acid sequence requires two features: an efficient conformational search algorithm and an energy function with a global minimum in the native state. As a step toward addressing both issues, a threadingbased method of secondary and tertiary restraint prediction has been developed and applied to ab initio folding. Such restraints are derived by extracting consensus contacts and local secondary structure from at least weakly scoring structures that, in some cases, can lack any global similarity to the sequence of interest. Furthermore, to generate representative protein structures, a reduced lattice-based protein model is used with replica exchange Monte Carlo to explore conformational space. We report results on the application of this methodology, termed TOUCHSTONE, to 65 proteins whose lengths range from 39 to 146 residues. For 47 (40) proteins, a cluster centroid whose rms deviation from native is below 6.5 (5) Å is found in one of the five lowest energy centroids. The number of correctly predicted proteins increases to 50 when atomic detail is added and a knowledge-based atomic potential is combined with clustered and nonclustered structures for candidate selection. The combination of the ratio of the relative number of contacts to the protein length and the number of clusters generated by the folding algorithm is a reliable indicator of the likelihood of successful fold prediction, thereby opening the way for genome-scale ab initio folding.T he inability to predict routinely the tertiary structure of a protein from its amino acid sequence remains one of the most challenging unsolved problems in biophysics. Contemporary approaches to this problem can be divided roughly into three categories of increasing complexity: (i) homology modeling (1, 2), (ii) threading (3, 4), and (iii) ab initio folding (5-9). The first two methods use the structures of already solved proteins as templates. The third, the ab initio method, does not require that an example of the fold of the protein of interest be previously solved. In principle, such an approach is very powerful; however, significant unresolved issues remain. First, there are problems with the search algorithms used to explore the protein's conformational space (10). Second, the energy functions used to evaluate the fitness of a given conformation cannot, in general, distinguish the native structure from alternative, protein-like decoys (11). To compensate for the imperfections in the energy functions, another way of selecting representative folds is required, with clustering of the structures being a promising approach (7-9). Finally, for a folding algorithm to be practical, one has to develop criteria that allow one to estimate the likelihood that a given prediction will be successful.In this article, we address each of these issues and present the results on the application of our ab initio method to a representative 65-protein test set. To restrict the protein's conformational space, we employ the SICHO (SId...