MACHINE LEARNING FOR PROTEIN CLASSIFICATION: KERNEL METHODSCS 374

Rajesh Ranganath

4/10/2008

OUTLINE

Biological Motivation and Background Algorithmic Concepts Mismatch Kernels Semi-supervised methods

PROTEINS

THE PROTEIN PROBLEM

Primary Structure can be easily determined 3D structure determines function Grouping proteins into structural and

evolutionary families is difficult Use machine learning to group proteins

HOW TO LOOK AT AMINO ACID CHAINS

Smith-Waterman Idea Mismatch Idea

FAMILIES

Proteins whose evolutionarily relationship is readily recognizable from the sequence (>~25% sequence identity)

Families are further subdivided into Proteins

Proteins are divided into Species The same protein may be found in

several species

Fold

Family

Superfamily

Proteins

Morten Nielsen,CBS, BioCentrum, DTU

SUPERFAMILIES

Proteins which are (remote) evolutionarily related

Sequence similarity low

Share function

Share special structural features

Relationships between members of a superfamily may not be readily recognizable from the sequence alone

Fold

Family

Superfamily

Proteins

Morten Nielsen,CBS, BioCentrum, DTU

FOLDS

Proteins which have >~50% secondary structure elements arranged the in the same order in the protein chain and in three dimensions are classified as having the same fold

No evolutionary relation between proteins

Fold

Family

Superfamily

Proteins

Morten Nielsen,CBS, BioCentrum, DTU

PROTEIN CLASSIFICATION Given a new protein, can we place it in its “correct”

position within an existing protein hierarchy?

Methods

BLAST / PsiBLAST

Profile HMMs

Supervised Machine Learning methods

Fold

Family

Superfamily

Proteins

?

new protein

MACHINE LEARNING CONCEPTS

Supervised Methods Discriminative Vs. Generative Models Transductive Learning Support Vector Machines Kernel Methods

Semi-supervised Methods

DISCRIMINATIVE AND GENERATIVE MODELS

Discriminative Generative

TRANSDUCTIVE LEARNING

Most Learning is Inductive Given (x1,y1) …. (xm,ym), for any test input x*

predict the label y* Transductive Learning

Given (x1,y1) …. (xm,ym) and all the test input {x1*,…, xp*} predict label {y1*,…, yp*}

SUPPORT VECTOR MACHINES

Popular Discriminative Learning algorithm Optimal geometric marginal classifier Can be solved efficiently using the Sequential

Minimal Optimization algorithm

If x1 … xn training examples, sign(iixiTx)

“decides” where x falls Train i to achieve best margin

SUPPORT VECTOR MACHINES (2)

Kernalizable: The SVM solution can be completely written down in terms of dot products of the input.

{sign(iiK(xi,x) determines class of x)}

KERNEL METHODS

K(x, z) = f(x)Tf(z) f is the feature mapping x and z are input vectors High dimensional features do not need to be

explicitly calculated Think of the kernel function similarity measure

between x and z

Example:

MISMATCH KERNEL

Regions of similar amino acid sequences yield a similar tertiary structure of proteins

Used as a kernel for an SVM to identify protein homologies

K-MER BASED SVMS For given word size k, and mismatch tolerance l,

define

K(X, Y) = # distinct k-long word occurrences with ≤ l mismatches

Define normalized mismatch kernel K’(X, Y) = K(X, Y)/ sqrt(K(X,X)K(Y,Y))

SVM can be learned by supplying this kernel functionA B A C A R D I

A B R A D A B I

X

Y

K(X, Y) = 4

K’(X, Y) = 4/sqrt(7*7) = 4/7 Let k = 3; l = 1

DISADVANTAGES

3D structure of proteins is practically impossible

Primary sequences are cheap to determine How do we use all this unlabeled data? Use semi-supervised learning based on the

cluster assumption

SEMI-SUPERVISED METHODS• Some examples are labeled

• Assume labels vary smoothly among all examples

• Some examples are labeled

• Assume labels vary smoothly among all examples

SEMI-SUPERVISED METHODS

• SVMs and other discriminative methods may make significant mistakes due to lack of data

SEMI-SUPERVISED METHODS• Some examples are labeled

• Assume labels vary smoothly among all examples

SEMI-SUPERVISED METHODS• Some examples are labeled

• Assume labels vary smoothly among all examples

SEMI-SUPERVISED METHODS• Some examples are labeled

• Assume labels vary smoothly among all examples

SEMI-SUPERVISED METHODS• Some examples are labeled

• Assume labels vary smoothly among all examples

Attempt to “contract” the distances within each cluster while keeping intracluster distances larger

SEMI-SUPERVISED METHODS• Some examples are labeled

• Assume labels vary smoothly among all examples

CLUSTER KERNELS Semi-supervised methods

1. Neighborhood 1. For each X, run PSI-BLAST to get similar seqs Nbd(X)

2. Define Φnbd(X) = 1/|Nbd(X)| X’ Nbd(X) Φoriginal(X’)

“Counts of all k-mers matching with at most 1 diff. all sequences that are similar to X”

3. Knbd(X, Y) = 1/(|Nbd(X)|*|Nbd(Y)) X’ Nbd(X) Y’ Nbd(Y) K(X’, Y’)

2. Next bagged mismatch

BAGGED MISMATCHED KERNEL Final method

1. Bagged mismatch

1. Run k-means clustering n times, giving p = 1,…,n assignments cp(X)

2. For every X and Y, count up the fraction of times they are bagged together

Kbag(X, Y) = 1/n p 1(cp(X) = cp (Y))

3. Combine the “bag fraction” with the original comparison K(.,.)

Knew(X, Y) = Kbag(X, Y) K(X, Y)

O. Jangmin

WHAT WORKS BEST?

Transductive Setting

REFERENCES

C. Leslie et al. Mismatch string kernels for discriminative protein classification. Bioinformatics Advance Access. January 22, 2004.

J. Weston et al. Semi-supervised protein classification using cluster kernels.2003.

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