Introduction to Bioinformatics Lecture 13: Predicting Protein Function Centre for Integrative...
-
date post
21-Dec-2015 -
Category
Documents
-
view
220 -
download
1
Transcript of Introduction to Bioinformatics Lecture 13: Predicting Protein Function Centre for Integrative...
Introduction to Bioinformatics
Lecture Lecture 1313: : Predicting Protein Function Predicting Protein Function
Centre for Centre for Integrative Bioinformatics VU (IBIVU)Integrative Bioinformatics VU (IBIVU)
The deluge of genomic information begs the following question: what do all these genes do?
Many genes are not annotated, and many more are partially or erroneously annotated. Given a genome which is partially annotated at best, how do we fill in the blanks?
Of each sequenced genome, 20%-50% of the functions of proteins encoded by the genomes remains unknown!
Protein Function Prediction
We are faced with the problem of predicting protein function from sequence, genomic, expression, interaction and structural data. For all these reasons and many more, automated protein function prediction is rapidly gaining interest among bioinformaticians and computational biologists
Protein Function Prediction
Outline Sequence-based function prediction
Structure-based function prediction– Sequence-structure comparison– Structure-structure comparison
Motif-based function prediction
Phylogenetic profile analysis
Protein interaction prediction and databases
Functional inference at systems level
Classes of function prediction methods Sequence based approaches
– protein A has function X, and protein B is a homolog (ortholog) of protein A; Hence B has function X
Structure-based approaches– protein A has structure X, and X has so-so structural features;
Hence A’s function sites are ….
Motif-based approaches– a group of genes have function X and they all have motif Y; protein
A has motif Y; Hence protein A’s function might be related to X
Function prediction based on “guilt-by-association”– gene A has function X and gene B is often “associated” with gene A,
B might have function related to X
Sequence-based function prediction Homology searching Sequence comparison is a powerful tool for detection
of homologous genes but limited to genomes that are not too distant away
uery: 2 LSDGEWQLVLNVWGKVEADIPGHGQEVLIRLFKGHPETLEKFDKFKHLKSEDEMKASEDL 61 LSD + V +W K+ G + L R+ +P+T F + D S ++Sbjct: 3 LSDKDKAAVRALWSKIGKSSDAIGNDALSRMIVVYPQTKIYFSHWP-----DVTPGSPNI 57
Query: 62 KKHGATVLTALGGILKKKGHHEAEIKPLAQSHATKHKIPVKYLEFISECIIQVLQSKHPG 121 K HG V+ + + K + + L++ HA K ++ + ++ CI+ V+ + PSbjct: 58 KAHGKKVMGGIALAVSKIDDLKTGLMELSEQHAYKLRVDPSNFKILNHCILVVISTMFPK 117
Query: 122 DFGADAQGAMNKALELFRKDMASNYK 147 +F +A +++K L +A Y+Sbjct: 118 EFTPEAHVSLDKFLSGVALALAERYR 143
We have done homology searching (FASTA, BLAST, PSI-BLAST) in earlier lectures
Structure-based function prediction
Structure-based methods could possibly detect remote homologues that are not detectable by sequence-based method– using structural information in addition to sequence
information– protein threading (sequence-structure alignment) is a
popular method
Structure-based methods could provide more than just “homology” information
Threading
Query sequence
Template sequence
+
Template structure
Compatibility score
Threading
Query sequence
Template sequence
+
Template structure
Compatibility score
Structure-based function prediction
Threading Scoring function for measuring to what extend query sequence fits into template structure
For scoring we have to map an amino acid (query sequence) onto a local environment (template structure)
We can use structural features for this:
o Secondary structure
o Is environment inside or outside? – Residue accessible surface area (ASA)
o Polarity of environment
The best (highest scoring) “thread” through the structure gives a so-called structural alignment, this looks exactly the same as a sequence alignment but is based on structure.
Fold recognition by threading
Query sequence
Compatibility scores
Fold 1
Fold 2
Fold 3
Fold N
Structure-based function prediction SCOP (http://scop.berkeley.edu/) is a protein structure
classification database where proteins are grouped into a hierarchy of families, superfamilies, folds and classes, based on their structural and functional similarities
Structure-based function prediction SCOP hierarchy – the top level: 11 classes
Structure-based function prediction
All-alpha protein
Coiled-coil proteinAll-beta protein
Alpha-beta proteinmembrane protein
Structure-based function prediction SCOP hierarchy – the second level: 800 folds
Structure-based function prediction SCOP hierarchy - third level: 1294 superfamilies
Structure-based function prediction
SCOP hierarchy - third level: 2327 families
Structure-based function prediction
Using sequence-structure alignment method, one can predict a protein belongs to a
– SCOP familiy, superfamily or fold
Proteins predicted to be in the same SCOP family are orthologous Proteins predicted to be in the same SCOPE superfamily are homologous Proteins predicted to be in the same SCOP fold are structurally
analogous
folds
superfamilies
families
Structure-based function prediction
Prediction of ligand binding sites– For ~85% of ligand-binding proteins, the largest largest cleft
is the ligand-binding site– For additional ~10% of ligand-binding proteins, the second
largest cleft is the ligand-binding site
Structure-based function prediction
Prediction of macromolecular binding site– there is a strong correlation between macromolecular
binding site (with protein, DNA and RNA) and disordered protein regions
– disordered regions in a protein sequence can be predicted using computational methods
http://www.pondr.com/
Motif-based function prediction
Prediction of protein functions based on identified sequence motifs
PROSITE contains patterns specific for more than a thousand protein families.
ScanPROSITE -- it allows to scan a protein sequence for occurrence of patterns and profiles stored in PROSITE
Motif-based function prediction
Search PROSITE using ScanPROSITE
The sequence has ASN_GLYCOSYLATION N-glycosylation site: 242 - 245 NETL
MSEGSDNNGDPQQQGAEGEAVGENKMKSRLRKGALKKKNVFNVKDHCFIARFFKQPTFCSHCKDFICGYQSGYAWMGFGKQGFQCQVCSYVVHKRCHEYVTFICPGKDKG IDSDSPKTQH ……..
Regular expressions
Alignment
ADLGAVFALCDRYFQSDVGPRSCFCERFYQADLGRTQNRCDRYYQADIGQPHSLCERYFQ
Regular expression
[AS]-D-[IVL]-G-x4-{PG}-C-[DE]-R-[FY]2-Q
{PG} = not (P or G)
For short sequence stretches, regular expressions are often more suitable to describe the information than alignments (or profiles)
Regular expressions
Regular expression No. of exact matches in DB
D-A-V-I-D 71
D-A-V-I-[DENQ] 252
[DENQ]-A-V-I-[DENQ] 925
[DENQ]-A-[VLI]-I-[DENQ] 2739
[DENQ]-[AG]-[VLI]2-[DENQ] 51506
D-A-V-E 1088
Phylogenetic profile analysis
Function prediction of genes based on “guilt-by-association” – a non-homologous approach
The phylogenetic profile of a protein is a string that encodes the presence or absence of the protein in every sequenced genome
Because proteins that participate in a common structural complex or metabolic pathway are likely to co-evolve, the phylogenetic profiles of such proteins are often ``similar''
Phylogenetic profile analysis
Phylogenetic profile (against N genomes)– For each gene X in a target genome (e.g., E coli),
build a phylogenetic profile as follows– If gene X has a homolog in genome #i, the ith bit
of X’s phylogenetic profile is “1” otherwise it is “0”
Phylogenetic profile analysis
Example – phylogenetic profiles based on 60 genomes
orf1034:1110110110010111110100010100000000111100011111110110111010101orf1036:1011110001000001010000010010000000010111101110011011010000101orf1037:1101100110000001110010000111111001101111101011101111000010100orf1038:1110100110010010110010011100000101110101101111111111110000101orf1039:1111111111111111111111111111111111111111101111111111111111101orf104: 1000101000000000000000101000000000110000000000000100101000100orf1040:1110111111111101111101111100000111111100111111110110111111101orf1041:1111111111111111110111111111111101111111101111111111111111101orf1042:1110100101010010010110000100001001111110111110101101100010101orf1043:1110100110010000010100111100100001111110101111011101000010101orf1044:1111100111110010010111010111111001111111111111101101100010101orf1045:1111110110110011111111111111111101111111101111111111110010101orf1046:0101100000010001011000000111110000010100000001010010100000000orf1047:0000000000000001000010000001000100000000000000010000000000000orf105: 0110110110100010111101101010111001101100101111100010000010001orf1054:0100100110000001100001000100000000100100100001000100100000000
Genes with similar phylogenetic profiles have related functions or functionally linked – D Eisenberg and colleagues (1999)
By correlating the rows (open reading frames (ORF) or genes) you find out about joint presence or absence of genes: this is a signal for a functional connection
gene
genome
Phylogenetic profile analysis
Phylogenetic profiles contain great amount of functional information
Phlylogenetic profile analysis can be used to distinguish orthologous genes from paralogous genes
Subcellular localization: 361 yeast nucleus-encoded mitochondrial proteins are identified at 50% accuracy with 58% coverage through phylogenetic profile analysis
Functional complementarity: By examining inverse phylogenetic profiles, one can find functionally complementary genes that have evolved through one of several mechanisms of convergent evolution.
Prediction of protein-protein interactions
Rosetta stone
Gene fusion is the an effective method for prediction of protein-protein interactions– If proteins A and B are homologous to two domains of a
protein C, A and B are predicted to have interaction
Though gene-fusion has low prediction coverage, it false-positive rate is low
A B
C
Domain fusion exampleVertebrates have a multi-enzyme protein (GARs-AIRs-GARt) comprising the enzymes GAR synthetase (GARs), AIR synthetase (AIRs), and GAR transformylase (GARt) 1. In insects, the polypeptide appears as GARs-(AIRs)2-GARt. However, GARs-AIRs is encoded separately from GARt in yeast, and in bacteria each domain is encoded separately (Henikoff et al., 1997).
1GAR: glycinamide ribonucleotide synthetase AIR: aminoimidazole ribonucleotide synthetase
Protein interaction database There are numerous databases of protein-protein
interactions
DIP is a popular protein-protein interaction database
The DIP database catalogs experimentally determined interactions between proteins. It combines information from a variety of sources to create a single, consistent set of protein-protein interactions.
Protein interaction databases
BIND - Biomolecular Interaction Network DatabaseDIP - Database of Interacting ProteinsPIM – HybrigenicsPathCalling Yeast Interaction Database MINT - a Molecular Interactions DatabaseGRID - The General Repository for Interaction DatasetsInterPreTS - protein interaction prediction through tertiary structureSTRING - predicted functional associations among genes/proteinsMammalian protein-protein interaction database (PPI)InterDom - database of putative interacting protein domains FusionDB - database of bacterial and archaeal gene fusion eventsIntAct ProjectThe Human Protein Interaction Database (HPID)ADVICE - Automated Detection and Validation of Interaction by Co-evolutionInterWeaver - protein interaction reports with online evidencePathBLAST - alignment of protein interaction networksClusPro - a fully automated algorithm for protein-protein dockingHPRD - Human Protein Reference Database
Protein interaction database
Network of protein interactions and predicted functional links involving silencing information regulator (SIR) proteins. Filled circles represent proteins of known function; open circles represent proteins of unknown function, represented only by their Saccharomyces genome sequence numbers ( http://genome-www.stanford.edu/Saccharomyces). Solid lines show experimentally determined interactions, as summarized in the Database of Interacting Proteins19 (http://dip.doe-mbi.ucla.edu). Dashed lines show functional links predicted by the Rosetta Stone method12. Dotted lines show functional links predicted by phylogenetic profiles16. Some predicted links are omitted for clarity.
Network of predicted functional linkages involving the yeast prion protein20 Sup35. The dashed line shows the only experimentally determined interaction. The other functional links were calculated from genome and expression data11 by a combination of methods, including phylogenetic profiles, Rosetta stone linkages and mRNA expression. Linkages predicted by more than one method, and hence particularly reliable, are shown by heavy lines. Adapted from ref. 11.
STRING - predicted functional associations among genes/proteins
STRING is a database of predicted functional associations among genes/proteins.
Genes of similar function tend to be maintained in close neighborhood, tend to be present or absent together, i.e. to have the same phylogenetic occurrence, and can sometimes be found fused into a single gene encoding a combined polypeptide.
STRING integrates this information from as many genomes as possible to predict functional links between proteins.
Berend Snel en Martijn Huynen (RUN) and the group of Peer Bork (EMBL, Heidelberg)
STRING - predicted functional associations among genes/proteins STRING is a database of known and predicted protein-protein interactions.The interactions include direct (physical) and indirect (functional) associations; they are derived from four sources:
1. Genomic Context (Synteny) 2. High-throughput Experiments 3. (Conserved) Co-expression 4. Previous Knowledge
STRING quantitatively integrates interaction data from these sources for a large number of organisms, and transfers information between these organisms where applicable. The database currently contains 736429 proteins in 179 species
STRING - predicted functional associations among genes/proteins
Conserved Neighborhood
This view shows runs of genes that occur repeatedly in close neighborhood in (prokaryotic) genomes. Genes located together in a run are linked with a black line (maximum allowed intergenic distance is 300 bp). Note that if there are multiple runs for a given species, these are separated by white space. If there are other genes in the run that are below the current score threshold, they are drawn as small white triangles. Gene fusion occurences are also drawn, but only if they are present in a run (see also the Fusion section below for more details).
Functional inference at systems level
Function prediction of individual genes could be made in the context of biological pathways/networks
Example – phoB is predicted to be a transcription regulator and it regulates all the genes in the pho-regulon (a group of co-regulated operons); and within this regulon, gene A is interacting with gene B, etc.
Functional inference at systems level
KEGG is database of biological pathways and networks
Functional inference at systems level
Functional inference at systems level
Functional inference at systems level
By doing homologous search, one can map a known biological pathway in one organism to another one; hence predict gene functions in the context of biological pathways/networks
Wrapping up
We have seen a number of ways to infer a putative function for a protein sequence
To gain confidence, it is important to combine as many different prediction protocols as possible (the STRING server is an example of this)
Homework
Give an example of two proteins having the same structural fold but different biological functions through searching SCOP and Swiss-prot
What is the biological function of phoR in the two-component system of prokaryotic organism based on KEGG database search