Summary: PH domain
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Pleckstrin homology domain Edit Wikipedia article
PH domain of tyrosine-protein kinase BTK
Pleckstrin homology domain (PH domain) is a protein domain of approximately 120 amino acids that occurs in a wide range of proteins involved in intracellular signaling or as constituents of the cytoskeleton.
This domain can bind Phosphatidylinositol lipids within biological membranes (such as Phosphatidylinositol (3,4,5)-trisphosphate and phosphatidylinositol (4,5)-bisphosphate), and proteins such as the βγ-subunits of heterotrimeric G proteins, and protein kinase C. Through these interactions, PH domains play a role in recruiting proteins to different membranes, thus targeting them to appropriate cellular compartments or enabling them to interact with other components of the signal transduction pathways.
Lipid binding specificity
Individual PH domains possess specificities for phosphoinositides phosphorylated at different sites within the inositol ring, e.g., some bind phosphatidylinositol (4,5)-bisphosphate but not phosphatidylinositol (3,4,5)-trisphosphate or phosphatidylinositol (3,4)-bisphosphate, while others may possess the requisite affinity. This is important because it makes the recruitment of different PH domain containing proteins sensitive to the activities of enzymes that either phosphorylate or dephosphorylate these sites on the inositol ring, such as phosphoinositide 3-kinase or PTEN, respectively. Thus, such enzymes exert a part of their effect on cell function by modulating the localization of downstream signaling proteins that possess PH domains that are capable of binding their phospholipid products.
The 3D structure of several PH domains has been determined. All known cases have a common structure consisting of two perpendicular anti-parallel beta sheets, followed by a C-terminal amphipathic helix. The loops connecting the beta-strands differ greatly in length, making the PH domain relatively difficult to detect while providing the source of the domain's specificity. The only conserved residue among PH domains is a single tryptophan located within the alpha helix that serves to nucleate the core of the domain.
Proteins containing PH domain
PH domains can be found in many different proteins, such as OSBP or ARF. Recruitment to the Golgi in this case is dependent on both PtdIns and ARF. A large number of PH domains have poor affinity for phosphoinositides and are hypothesized to function as protein binding domains. A Genome-wide look in Saccharomyces cerevisiae showed that most of the 33 yeast PH domains are indeed promiscuous in binding to phosphoinositides, while only one (Num1-PH) behaved highly specific . Proteins reported to contain PH domains belong to the following families:
- Pleckstrin, the protein where this domain was first detected, is the major substrate of protein kinase C in platelets. Pleckstrin is one of the rare proteins to contain two PH domains.
- Ser/Thr protein kinases such as the Akt/Rac family, the beta-adrenergic receptor kinases, the mu isoform of PKC and the trypanosomal NrkA family.
- Tyrosine protein kinases belonging to the Btk/Itk/Tec subfamily.
- Insulin Receptor Substrate 1 (IRS-1).
- Regulators of small G-proteins like guanine nucleotide releasing factor GNRP (Ras-GRF) (which contains 2 PH domains), guanine nucleotide exchange proteins like vav, dbl, SoS and S. cerevisiae CDC24, GTPase activating proteins like rasGAP and BEM2/IPL2, and the human break point cluster protein bcr.
- Cytoskeletal proteins such as dynamin (see IPR001401), Caenorhabditis elegans kinesin-like protein unc-104 (see IPR001752), spectrin beta-chain, syntrophin (2 PH domains), and S. cerevisiae nuclear migration protein NUM1.
- Mammalian phosphatidylinositol-specific phospholipase C (PI-PLC) (see IPR000909) isoforms gamma and delta. Isoform gamma contains two PH domains, the second one is split into two parts separated by about 400 residues.
- Oxysterol-binding proteins OSBP, S. cerevisiae OSH1 and YHR073w.
- Mouse protein citron, a putative rho/rac effector that binds to the GTP-bound forms of rho and rac.
- Several S. cerevisiae proteins involved in cell cycle regulation and bud formation like BEM2, BEM3, BUD4 and the BEM1-binding proteins BOI2 (BEB1) and BOI1 (BOB1).
- C. elegans protein MIG-10.
- Ceramide kinase, a lipid kinase that phosphorylates ceramides to ceramide-1-phosphate.
Human genes encoding proteins containing this domain include:
- ABR, ADRBK1, ADRBK2, AFAP, AFAP1, AFAP1L1, AFAP1L2, AKAP13, AKT1, AKT2, AKT3, ANLN, APBB1IP, APPL1, APPL2, ARHGAP10, ARHGAP12, ARHGAP15, ARHGAP21, ARHGAP22, ARHGAP23, ARHGAP24, ARHGAP25, ARHGAP26, ARHGAP27, ARHGAP9, ARHGEF16, ARHGEF18, ARHGEF19, ARHGEF2, ARHGEF3, ARHGEF4, ARHGEF5, ARHGEF6, ARHGEF7, ARHGEF9, ASEF2,
- BMX, BTK,
- C20orf42, C9orf100, CADPS, CADPS2, CDC42BPA, CDC42BPB, CDC42BPG, CENTA1, CENTA2, CENTB1, CENTB2, CENTB5, CENTD1, CENTD2, CENTD3, CENTG1, CENTG2, CENTG3, CIT, CNKSR1, CNKSR2, COL4A3BP, CTGLF1, CTGLF2, CTGLF3, * CTGLF4, CTGLF5, CTGLF6,
- DAB2IP, DAPP1, DDEF1, DDEF2, DDEFL1, DEF6, DEPDC2, DGKD, DGKH, DGKK, DNM1, DNM2, DNM3, DOCK10, DOCK11, DOCK9, DOK1, DOK2, DOK3, DOK4, DOK5, DOK6, DTGCU2,
- FAM109A, FAM109B, FARP1, FARP2, FGD1, FGD2, FGD3, FGD4, FGD5, FGD6,
- GAB1, GAB2, GAB3, GAB4, GRB10, GRB14, GRB7,
- IRS1, IRS2, IRS4, ITK, ITSN1, ITSN2,
- KALRN, KIF1A, KIF1B, KIF1Bbeta,
- MCF2, MCF2L, MCF2L2, MRIP, MYO10,
- NET1, NGEF,
- OBPH1, OBSCN, OPHN1, OSBP, OSBP2, OSBPL10, OSBPL11, OSBPL3, OSBPL5, OSBPL6, OSBPL7, OSBPL8, OSBPL9,
- PHLDA2, PHLDA3, PHLDB1, PHLDB2, PHLPP, PIP3-E, PLCD1, PLCD4, PLCG1, PLCG2, PLCH1, PLCH2, PLCL1, PLCL2, PLD1, PLD2, PLEK, PLEK2, PLEKHA1, PLEKHA2, PLEKHA3, PLEKHA4, PLEKHA5, PLEKHA6, PLEKHA7, PLEKHA8, PLEKHB1, PLEKHB2, PLEKHC1, PLEKHF1, PLEKHF2, PLEKHG1, PLEKHG2, PLEKHG3, PLEKHG4, PLEKHG5, PLEKHG6, PLEKHH1, PLEKHH2, PLEKHH3, PLEKHJ1, PLEKHK1, PLEKHM1, PLEKHM2, PLEKHO1, PLEKHQ1, PREX1, PRKCN, PRKD1, PRKD2, PRKD3, PSCD1, PSCD2, PSCD3, PSCD4, PSD, PSD2, PSD3, PSD4, RALGPS1, RALGPS2, RAPH1,
- RASA1, RASA2, RASA3, RASA4, RASAL1, RASGRF1, RGNEF, ROCK1, ROCK2, RTKN,
- SBF1, SBF2, SCAP2, SGEF, SH2B, SH2B1, SH2B2, SH2B3, SH3BP2, SKAP1, SKAP2, SNTA1, SNTB1, SNTB2, SOS1, SOS2, SPATA13, SPNB4, SPTBN1, SPTBN2, SPTBN4, SPTBN5, STAP1, SWAP70, SYNGAP1,
- TBC1D2, TEC, TIAM1, TRIO, TRIOBP, TYL,
- URP1, URP2,
- VAV1, VAV2, VAV3, VEPH1
- The unrelated FYVE domain binds Phosphatidylinositol 3-phosphate and has been found in over 60 proteins.
- The GRAM domain is a structurally related protein domain.
- Mayer, B. J.; Ren, R.; Clark, K. L.; Baltimore, D. (1993). "A putative modular domain present in diverse signaling proteins". Cell 73 (4): 629–630. doi:10.1016/0092-8674(93)90244-K. PMID 8500161.
- Haslam, R. J.; Koide, H. B.; Hemmings, B. A. (1993). "Pleckstrin domain homology". Nature 363 (6427): 309. doi:10.1038/363309b0.
- Musacchio, A.; Gibson, T.; Rice, P.; Thompson, J.; Saraste, M. (1993). "The PH domain: A common piece in the structural pathcwork of signalling proteins". Trends in Biochemical Sciences 18 (9): 343–348. doi:10.1016/0968-0004(93)90071-T. PMID 8236453.
- Gibson, T. J.; Hyvönen, M.; Musacchio, A.; Saraste, M.; Birney, E. (1994). "PH domain: The first anniversary". Trends in Biochemical Sciences 19 (9): 349–353. doi:10.1016/0968-0004(94)90108-2. PMID 7985225.
- Pawson, T. (1995). "Protein modules and signalling networks". Nature 373 (6515): 573–580. doi:10.1038/373573a0. PMID 7531822.
- Ingley, E.; Hemmings, B. A. (1994). "Pleckstrin homology (PH) domains in signal transducton". Journal of Cellular Biochemistry 56 (4): 436–443. doi:10.1002/jcb.240560403. PMID 7890802.
- Saraste, M.; Hyvönen, M. (1995). "Pleckstrin homology domains: A fact file". Current Opinion in Structural Biology 5 (3): 403–408. doi:10.1016/0959-440X(95)80104-9. PMID 7583640.
- Wang, D. S.; Shaw, G. (1995). "The Association of the C-Terminal Region of β1ΣII Spectrin to Brain Membranes is Mediated by a pH Domain, Does Not Require Membrane Proteins, and Coincides with a Inositol-1,4,5 Trisphosphate Binding Site". Biochemical and Biophysical Research Communications 217 (2): 608–615. doi:10.1006/bbrc.1995.2818. PMID 7503742.
- Wang, D. S.; Shaw, R.; Winkelmann, J. C.; Shaw, G. (1994). "Binding of PH Domains of β-Adrenergic-Receptor Kinase and β-Spectrin to WD40/β-Transducin Repeat Containing Regions of the β-Subunit of Trimeric G-Proteins". Biochemical and Biophysical Research Communications 203 (1): 29–35. doi:10.1006/bbrc.1994.2144. PMID 8074669.
- Yao, L.; Kawakami, Y.; Kawakami, T. (1994). "The pleckstrin homology domain of Bruton tyrosine kinase interacts with protein kinase C". Proceedings of the National Academy of Sciences 91 (19): 9175. doi:10.1073/pnas.91.19.9175.
- Riddihough, G. (1994). "More meanders and sandwiches". Nature Structural Biology 1 (11): 755–757. doi:10.1038/nsb1194-755. PMID 7634082.
- Yu, J. W.; Mendrola, J. M.; Audhya, A.; Singh, S.; Keleti, D.; Dewald, D. B.; Murray, D.; Emr, S. D.; Lemmon, M. A. (2004). "Genome-Wide Analysis of Membrane Targeting by S. Cerevisiae Pleckstrin Homology Domains". Molecular Cell 13 (5): 677–688. doi:10.1016/S1097-2765(04)00083-8. PMID 15023338.
- Sugiura, M.; Kono, K.; Liu, H.; Shimizugawa, T.; Minekura, H.; Spiegel, S.; Kohama, T. (2002). "Ceramide Kinase, a Novel Lipid Kinase. MOLECULAR CLONING AND FUNCTIONAL CHARACTERIZATION". Journal of Biological Chemistry 277 (26): 23294–23300. doi:10.1074/jbc.M201535200. PMID 11956206.
- Nash Lab Protein Interaction Domains - PH domain description
- UMich Orientation of Proteins in Membranes families/superfamily-51 - Calculated orientations of PH domains in membranes
This tab holds the annotation information that is stored in the Pfam database. As we move to using Wikipedia as our main source of annotation, the contents of this tab will be gradually replaced by the Wikipedia tab.
PH domain Provide feedback
PH stands for pleckstrin homology.
Internal database links
|Similarity to PfamA using HHSearch:||Mcp5_PH PH_3 PH_4 PH_5 PH_6 PH_7 PH_8 PH_9 PH_10 PH_11|
External database links
This tab holds annotation information from the InterPro database.
InterPro entry IPR001849
Pleckstrin homology (PH) domains are small modular domains that occur in a large variety of proteins. The domains can bind phosphatidylinositol within biological membranes and proteins such as the beta/gamma subunits of heterotrimeric G proteins [PUBMED:8074669] and protein kinase C [PUBMED:7522330]. Through these interactions, PH domains play a role in recruiting proteins to different membranes, thus targeting them to appropriate cellular compartments or enabling them to interact with other components of the signal transduction pathways.
PH domains have been found to possess inserted domains (such as in PLC gamma, syntrophins) and to be inserted within other domains. Mutations in Brutons tyrosine kinase (Btk) within its PH domain cause X-linked agammaglobulinaemia (XLA) in patients. Point mutations cluster into the positively charged end of the molecule around the predicted binding site for phosphatidylinositol lipids.
The 3D structure of several PH domains has been determined [PUBMED:7634082]. All known cases have a common structure consisting of two perpendicular anti-parallel beta sheets, followed by a C-terminal amphipathic helix. The loops connecting the beta-strands differ greatly in length, making the PH domain relatively difficult to detect. There are no totally invariant residues within the PH domain.
Proteins reported to contain one more PH domains belong to the following families:
- Pleckstrin, the protein where this domain was first detected, is the major substrate of protein kinase C in platelets. Pleckstrin is one of the rare proteins to contains two PH domains.
- Ser/Thr protein kinases such as the Akt/Rac family, the beta-adrenergic receptor kinases, the mu isoform of PKC and the trypanosomal NrkA family.
- Tyrosine protein kinases belonging to the Btk/Itk/Tec subfamily.
- Insulin Receptor Substrate 1 (IRS-1).
- Regulators of small G-proteins like guanine nucleotide releasing factor GNRP (Ras-GRF) (which contains 2 PH domains), guanine nucleotide exchange proteins like vav, dbl, SoS and Saccharomyces cerevisiae CDC24, GTPase activating proteins like rasGAP and BEM2/IPL2, and the human break point cluster protein bcr.
- Cytoskeletal proteins such as dynamin (see INTERPRO), Caenorhabditis elegans kinesin-like protein unc-104 (see INTERPRO), spectrin beta-chain, syntrophin (2 PH domains) and S. cerevisiae nuclear migration protein NUM1.
- Mammalian phosphatidylinositol-specific phospholipase C (PI-PLC) (see INTERPRO) isoforms gamma and delta. Isoform gamma contains two PH domains, the second one is split into two parts separated by about 400 residues.
- Oxysterol binding proteins OSBP, S. cerevisiae OSH1 and YHR073w.
- Mouse protein citron, a putative rho/rac effector that binds to the GTP-bound forms of rho and rac. Several S. cerevisiae proteins involved in cell cycle regulation and bud formation like BEM2, BEM3, BUD4 and the BEM1-binding proteins BOI2 (BEB1) and BOI1 (BOB1).
- C. elegans protein MIG-10.
- C. elegans hypothetical proteins C04D8.1, K06H7.4 and ZK632.12.
- S. cerevisiae hypothetical proteins YBR129c and YHR155w.
The mapping between Pfam and Gene Ontology is provided by InterPro. If you use this data please cite InterPro.
|Molecular function||protein binding (GO:0005515)|
|phospholipid binding (GO:0005543)|
Below is a listing of the unique domain organisations or architectures in which this domain is found. More...
The graphic that is shown by default represents the longest sequence with a given architecture. Each row contains the following information:
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a textual description of the architecture, e.g. Gla, EGF x 2, Trypsin.
This example describes an architecture with one
Gladomain, followed by two consecutive
EGFdomains, and finally a single
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Note that you can see the family page for a particular domain by clicking on the graphic. You can also choose to see all sequences which have a given architecture by clicking on the Show link in each row.
Finally, because some families can be found in a very large number of architectures, we load only the first fifty architectures by default. If you want to see more architectures, click the button at the bottom of the page to load the next set.
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Members of this clan share a PH-like fold. Many families in this clan bind to short peptide motifs in proteins and are involved in signalling.
The clan contains the following 33 members:bPH_1 bPH_2 bPH_3 bPH_4 bPH_5 bPH_6 DCP1 DUF1448 FERM_C GRAM ICAP-1_inte_bdg Mcp5_PH PH PH_10 PH_11 PH_2 PH_3 PH_4 PH_5 PH_6 PH_7 PH_8 PH_9 PH_BEACH PID PID_2 PTB Ran_BP1 Rtt106 SSrecog Voldacs Vps36_ESCRT-II WH1
We store a range of different sequence alignments for families. As well as the seed alignment from which the family is built, we provide the full alignment, generated by searching the sequence database using the family HMM. We also generate alignments using four representative proteomes (RP) sets, the NCBI sequence database, and our metagenomics sequence database. More...
There are various ways to view or download the sequence alignments that we store. We provide several sequence viewers and a plain-text Stockholm-format file for download.
We make a range of alignments for each Pfam-A family:
- the curated alignment from which the HMM for the family is built
- the alignment generated by searching the sequence database using the HMM
- Representative Proteomes (RPs) at 15%, 35%, 55% and 75% co-membership thresholds
- alignment generated by searching the NCBI sequence database using the family HMM
- alignment generated by searching the metagenomics sequence database using the family HMM
You can see the alignments as HTML or in three different sequence viewers:
- a Java applet developed at the University of Dundee. You will need Java installed before running jalview
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We make a range of alignments for each Pfam-A family. You can see a description of each above. You can view these alignments in various ways but please note that some types of alignment are never generated while others may not be available for all families, most commonly because the alignments are too large to handle.
1Cannot generate PP/Heatmap alignments for seeds; no PP data available
Key: available, not generated, — not available.
Format an alignment
We make all of our alignments available in Stockholm format. You can download them here as raw, plain text files or as gzip-compressed files.
You can also download a FASTA format file containing the full-length sequences for all sequences in the full alignment.
MyHits provides a collection of tools to handle multiple sequence alignments. For example, one can refine a seed alignment (sequence addition or removal, re-alignment or manual edition) and then search databases for remote homologs using HMMER3.
HMM logos is one way of visualising profile HMMs. Logos provide a quick overview of the properties of an HMM in a graphical form. You can see a more detailed description of HMM logos and find out how you can interpret them here. More...
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This page displays the phylogenetic tree for this family's seed alignment. We use FastTree to calculate neighbour join trees with a local bootstrap based on 100 resamples (shown next to the tree nodes). FastTree calculates approximately-maximum-likelihood phylogenetic trees from our seed alignment.
Note: You can also download the data file for the tree.
Curation and family details
This section shows the detailed information about the Pfam family. You can see the definitions of many of the terms in this section in the glossary and a fuller explanation of the scoring system that we use in the scores section of the help pages.
|Number in seed:||133|
|Number in full:||16347|
|Average length of the domain:||107.30 aa|
|Average identity of full alignment:||17 %|
|Average coverage of the sequence by the domain:||13.94 %|
|HMM build commands:||
build method: hmmbuild -o /dev/null HMM SEED
search method: hmmsearch -Z 23193494 -E 1000 --cpu 4 HMM pfamseq
|Family (HMM) version:||24|
|Download:||download the raw HMM for this family|
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This visualisation provides a simple graphical representation of the distribution of this family across species. You can find the original interactive tree in the More....
This chart is a modified "sunburst" visualisation of the species tree for this family. It shows each node in the tree as a separate arc, arranged radially with the superkingdoms at the centre and the species arrayed around the outermost ring.
How the sunburst is generated
The tree is built by considering the taxonomic lineage of each sequence that has a match to this family. For each node in the resulting tree, we draw an arc in the sunburst. The radius of the arc, its distance from the root node at the centre of the sunburst, shows the taxonomic level ("superkingdom", "kingdom", etc). The length of the arc represents either the number of sequences represented at a given level, or the number of species that are found beneath the node in the tree. The weighting scheme can be changed using the sunburst controls.
In order to reduce the complexity of the representation, we reduce the number of taxonomic levels that we show. We consider only the following eight major taxonomic levels:
Colouring and labels
Segments of the tree are coloured approximately according to their superkingdom. For example, archeal branches are coloured with shades of orange, eukaryotes in shades of purple, etc. The colour assignments are shown under the sunburst controls. Where space allows, the name of the taxonomic level will be written on the arc itself.
As you move your mouse across the sunburst, the current node will be highlighted. In the top section of the controls panel we show a summary of the lineage of the currently highlighed node. If you pause over an arc, a tooltip will be shown, giving the name of the taxonomic level in the title and a summary of the number of sequences and species below that node in the tree.
Anomalies in the taxonomy tree
There are some situations that the sunburst tree cannot easily handle and for which we have work-arounds in place.
Missing taxonomic levels
Some species in the taxonomic tree may not have one or more of the main eight levels that we display. For example, Bos taurus is not assigned an order in the NCBI taxonomic tree. In such cases we mark the omitted level with, for example, "No order", in both the tooltip and the lineage summary.
Unmapped species names
The tree is built by looking at each sequence in the full alignment for the family. We take the name of the species given by UniProt and try to map that to the full taxonomic tree from NCBI. In some cases, the name chosen by UniProt does not map to any node in the NCBI tree, perhaps because the chosen name is listed as a synonym or a misspelling in the NCBI taxonomy.
So that these nodes are not simply omitted from the sunburst tree, we group them together in a separate branch (or segment of the sunburst tree). Since we cannot determine the lineage for these unmapped species, we show all levels between the superkingdom and the species as "uncategorised".
Since we reduce the species tree to only the eight main taxonomic levels, sequences that are mapped to the sub-species level in the tree would not normally be shown. Rather than leave out these species, we map them instead to their parent species. So, for example, for sequences belonging to one of the Vibrio cholerae sub-species in the NCBI taxonomy, we show them instead as belonging to the species Vibrio cholerae.
Too many species/sequences
For large species trees, you may see blank regions in the outer layers of the sunburst. These occur when there are large numbers of arcs to be drawn in a small space. If an arc is less than approximately one pixel wide, it will not be drawn and the space will be left blank. You may still be able to get some information about the species in that region by moving your mouse across the area, but since each arc will be very small, it will be difficult to accurately locate a particular species.
The tree shows the occurrence of this domain across different species. More...
We show the species tree in one of two ways. For smaller trees we try to show an interactive representation, which allows you to select specific nodes in the tree and view them as an alignment or as a set of Pfam domain graphics.
Unfortunately we have found that there are problems viewing the interactive tree when the it becomes larger than a certain limit. Furthermore, we have found that Internet Explorer can become unresponsive when viewing some trees, regardless of their size. We therefore show a text representation of the species tree when the size is above a certain limit or if you are using Internet Explorer to view the site.
If you are using IE you can still load the interactive tree by clicking the "Generate interactive tree" button, but please be aware of the potential problems that the interactive species tree can cause.
For all of the domain matches in a full alignment, we count the number that are found on all sequences in the alignment. This total is shown in the purple box.
We also count the number of unique sequences on which each domain is found, which is shown in green. Note that a domain may appear multiple times on the same sequence, leading to the difference between these two numbers.
Finally, we group sequences from the same organism according to the NCBI code that is assigned by UniProt, allowing us to count the number of distinct sequences on which the domain is found. This value is shown in the pink boxes.
We use the NCBI species tree to group organisms according to their taxonomy and this forms the structure of the displayed tree. Note that in some cases the trees are too large (have too many nodes) to allow us to build an interactive tree, but in most cases you can still view the tree in a plain text, non-interactive representation. Those species which are represented in the seed alignment for this domain are highlighted.
You can use the tree controls to manipulate how the interactive tree is displayed:
- show/hide the summary boxes
- highlight species that are represented in the seed alignment
- expand/collapse the tree or expand it to a given depth
- select a sub-tree or a set of species within the tree and view them graphically or as an alignment
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Please note: for large trees this can take some time. While the tree is loading, you can safely switch away from this tab but if you browse away from the family page entirely, the tree will not be loaded.
There are 7 interactions for this family. More...
We determine these interactions using iPfam, which considers the interactions between residues in three-dimensional protein structures and maps those interactions back to Pfam families. You can find more information about the iPfam algorithm in the journal article that accompanies the website.
For those sequences which have a structure in the Protein DataBank, we use the mapping between UniProt, PDB and Pfam coordinate systems from the PDBe group, to allow us to map Pfam domains onto UniProt sequences and three-dimensional protein structures. The table below shows the structures on which the PH domain has been found. There are 205 instances of this domain found in the PDB. Note that there may be multiple copies of the domain in a single PDB structure, since many structures contain multiple copies of the same protein seqence.
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