Bioinformatics Advance Access originally published online on January 22, 2004
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Bioinformatics 20(4) © Oxford University Press 2004; all rights reserved.
Mismatch string kernels for discriminative protein classification

1 Department of Computer Science, Columbia University, 1214 Amsterdam Avenue, Mail Code 0401, New York, NY 10027, USA, 2 Max-Planck Institute for Biological Cybernetics, Spemannstrasse 38, 72076 Tübingen, Germany and 3 Department of Genome Sciences, University of Washington, 1705 NE Pacific Street, Seattle, WA 98195, USA
Received on February 19, 2003
; revised on June 21, 2003
; accepted on August 5, 2003
Advance Access Publication January 22, 2004
Motivation: Classification of proteins sequences into functional and structural families based on sequence homology is a central problem in computational biology. Discriminative supervised machine learning approaches provide good performance, but simplicity and computational efficiency of training and prediction are also important concerns.
Results: We introduce a class of string kernels, called mismatch kernels, for use with support vector machines (SVMs) in a discriminative approach to the problem of protein classification and remote homology detection. These kernels measure sequence similarity based on shared occurrences of fixed-length patterns in the data, allowing for mutations between patterns. Thus, the kernels provide a biologically well-motivated way to compare protein sequences without relying on family-based generative models such as hidden Markov models. We compute the kernels efficiently using a mismatch tree data structure, allowing us to calculate the contributions of all patterns occurring in the data in one pass while traversing the tree. When used with an SVM, the kernels enable fast prediction on test sequences. We report experiments on two benchmark SCOP datasets, where we show that the mismatch kernel used with an SVM classifier performs competitively with state-of-the-art methods for homology detection, particularly when very few training examples are available. Examination of the highest-weighted patterns learned by the SVM classifier recovers biologically important motifs in protein families and superfamilies.
Availability: SVM software is publicly available at http://microarray.cpmc.columbia.edu/gist. Mismatch kernel software is available upon request.
Contact: cleslie{at}cs.columbia.edu
* To whom correspondence should be addressed.
Formerly William Noble Grundy, see www.gs.washington.edu/noble/name-change.html
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