Computational Biology and Computer Science Departments, School of Computer Science, Carnegie Mellon University, Pittsburgh, PA, USA.|Computational Biology and Computer Science Departments, School of Computer Science, Carnegie Mellon University, Pittsburgh, PA, USA.|Department of Molecular Structural Biology, Max Planck Institute of Biochemistry, Martinsried, Germany.|Computational Biology and Computer Science Departments, School of Computer Science, Carnegie Mellon University, Pittsburgh, PA, USA.