Summary: Cytochrome oxidase assembly protein
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Cytochrome c oxidase Edit Wikipedia article
|Cytochrome C Oxidase|
The crystal structure of bovine cytochrome c oxidase in a phospholipid bilayer. The intermembrane space lies to top of the image. Adapted from (It is a homo dimer in this structure)
|PDB structures||RCSB PDB PDBe PDBsum|
|Gene Ontology||AmiGO / EGO|
It is the last enzyme in the respiratory electron transport chain of mitochondria (or bacteria) located in the mitochondrial (or bacterial) membrane. It receives an electron from each of four cytochrome c molecules, and transfers them to one oxygen molecule, converting molecular oxygen to two molecules of water. In the process, it binds four protons from the inner aqueous phase to make water, and in addition translocates four protons across the membrane, helping to establish a transmembrane difference of proton electrochemical potential that the ATP synthase then uses to synthesize ATP.
- 1 Structure
- 2 Assembly
- 3 Table of conserved subunits of cytochrome c oxidase complex
- 4 Biochemistry
- 5 Inhibition
- 6 Subcellular localization and presence at extramitochondrial sites
- 7 Genetic defects and disorders
- 8 Histochemistry
- 9 Additional images
- 10 See also
- 11 References
- 12 External links
The complex is a large integral membrane protein composed of several metal prosthetic sites and 14  protein subunits in mammals. In mammals, eleven subunits are nuclear in origin, and three are synthesized in the mitochondria. The complex contains two hemes, a cytochrome a and cytochrome a3, and two copper centers, the CuA and CuB centers. In fact, the cytochrome a3 and CuB form a binuclear center that is the site of oxygen reduction. Cytochrome c, which is reduced by the preceding component of the respiratory chain (cytochrome bc1 complex, complex III), docks near the CuA binuclear center and passes an electron to it, being oxidized back to cytochrome c containing Fe3+. The reduced CuA binuclear center now passes an electron on to cytochrome a, which in turn passes an electron on to the cytochrome a3-CuB binuclear center. The two metal ions in this binuclear center are 4.5 Å apart and coordinate a hydroxide ion in the fully oxidized state.
Crystallographic studies of cytochrome c oxidase show an unusual post-translational modification, linking C6 of Tyr(244) and the ε-N of His(240) (bovine enzyme numbering). It plays a vital role in enabling the cytochrome a3- CuB binuclear center to accept four electrons in reducing molecular oxygen to water. The mechanism of reduction was formerly thought to involve a peroxide intermediate, which was believed to lead to superoxide production. However, the currently accepted mechanism involves a rapid four-electron reduction involving immediate oxygen-oxygen bond cleavage, avoiding any intermediate likely to form superoxide.
COX assembly in yeast is a complex process that is not entirely understood due to the rapid and irreversible aggregation of hydrophobic subunits that form the holoenzyme complex, as well as aggregation of mutant subunits with exposed hydrophobic patches. COX subunits are encoded in both the nuclear and mitochondrial genomes. The three subunits that form the COX catalytic core are encoded in the mitochondrial genome.
Hemes and cofactors are inserted into subunits I & II. Subunits I and IV initiate assembly. Different subunits may associate to form sub-complex intermediates that later bind to other subunits to form the COX complex. In post-assembly modifications, COX will form a homodimer. This is required for activity. Both dimers are connected by a cardiolipin molecule, which has been found to play a key role in stabilization of the holoenzyme complex. The dissociation of subunits VIIa and III in conjunction with the removal of cardiolipin results in total loss of enzyme activity. Subunits encoded in the nuclear genome are known to play a role in enzyme dimerization and stability. Mutations to these subunits eliminate COX function.
Assembly is known to occur in at least three distinct rate-determining steps. The products of these steps have been found, though specific subunit compositions have not been determined.
Synthesis and assembly of COX subunits I, II, and III are facilitated by translational activators, which interact with the 5’ untranslated regions of mitochondrial mRNA transcripts. Translational activators are encoded in the nucleus. They can operate through either direct or indirect interaction with other components of translation machinery, but exact molecular mechanisms are unclear due to difficulties associated with synthesizing translation machinery in-vitro. Though the interactions between subunits I, II, and III encoded within the mitochondrial genome make a lesser contribution to enzyme stability than interactions between bigenomic subunits, these subunits are more conserved, indicating potential unexplored roles for enzyme activity.
|No.||Subunit name||Human protein||Protein description from UniProt||Pfam family with Human protein|
|1||Cox1||COX1_HUMAN||Cytochrome c oxidase subunit 1||Pfam PF00115|
|2||Cox2||COX2_HUMAN||Cytochrome c oxidase subunit 2||Pfam PF02790, Pfam PF00116|
|3||Cox3||COX3_HUMAN||Cytochrome c oxidase subunit 3||Pfam PF00510|
|4||Cox4i1||COX41_HUMAN||Cytochrome c oxidase subunit 4 isoform 1, mitochondrial||Pfam PF02936|
|5||Cox4a2||COX42_HUMAN||Cytochrome c oxidase subunit 4 isoform 2, mitochondrial||Pfam PF02936|
|6||Cox5a||COX5A_HUMAN||Cytochrome c oxidase subunit 5A, mitochondrial||Pfam PF02284|
|7||Cox5b||COX5B_HUMAN||Cytochrome c oxidase subunit 5B, mitochondrial||Pfam PF01215|
|8||Cox6a1||CX6A1_HUMAN||Cytochrome c oxidase subunit 6A1, mitochondrial||Pfam PF02046|
|9||Cox6a2||CX6A2_HUMAN||Cytochrome c oxidase subunit 6A2, mitochondrial||Pfam PF02046|
|10||Cox6b1||CX6B1_HUMAN||Cytochrome c oxidase subunit 6B1||Pfam PF02297|
|11||Cox6b2||CX6B2_HUMAN||Cytochrome c oxidase subunit 6B2||Pfam PF02297|
|12||Cox6c||COX6C_HUMAN||Cytochrome c oxidase subunit 6C||Pfam PF02937|
|13||Cox7a1||CX7A1_HUMAN||Cytochrome c oxidase subunit 7A1, mitochondrial||Pfam PF02238|
|14||Cox7a2||CX7A2_HUMAN||Cytochrome c oxidase subunit 7A2, mitochondrial||Pfam PF02238|
|15||Cox7a3||COX7S_HUMAN||Putative cytochrome c oxidase subunit 7A3, mitochondrial||Pfam PF02238|
|16||Cox7b||COX7B_HUMAN||Cytochrome c oxidase subunit 7B, mitochondrial||Pfam PF05392|
|17||Cox7c||COX7C_HUMAN||Cytochrome c oxidase subunit 7C, mitochondrial||Pfam PF02935|
|18||Cox7r||COX7R_HUMAN||Cytochrome c oxidase subunit 7A-related protein, mitochondrial||Pfam PF02238|
|19||Cox8a||COX8A_HUMAN||Cytochrome c oxidase subunit 8A, mitochondrial P||Pfam PF02285|
|20||Cox8c||COX8C_HUMAN||Cytochrome c oxidase subunit 8C, mitochondrial||Pfam PF02285|
|1||Coa1||COA1_HUMAN||Cytochrome c oxidase assembly factor 1 homolog||Pfam PF08695|
|2||Coa3||COA3_HUMAN||Cytochrome c oxidase assembly factor 3 homolog, mitochondrial||Pfam PF09813|
|3||Coa4||COA4_HUMAN||Cytochrome c oxidase assembly factor 4 homolog, mitochondrial||Pfam PF06747|
|4||Coa5||COA5_HUMAN||Cytochrome c oxidase assembly factor 5||Pfam PF10203|
|5||Coa6||COA6_HUMAN||Cytochrome c oxidase assembly factor 6 homolog||Pfam PF02297|
|6||Coa7||COA7_HUMAN||Cytochrome c oxidase assembly factor 7,||Pfam PF08238|
|7||Cox11||COX11_HUMAN||Cytochrome c oxidase assembly protein COX11 mitochondrial||Pfam PF04442|
|8||Cox14||COX14_HUMAN||Cytochrome c oxidase assembly protein||Pfam PF14880|
|9||Cox15||COX15_HUMAN||Cytochrome c oxidase assembly protein COX15 homolog||Pfam PF02628|
|10||Cox16||COX16_HUMAN||Cytochrome c oxidase assembly protein COX16 homolog mitochondrial||Pfam PF14138|
|11||Cox17||COX17_HUMAN||Cytochrome c oxidase copper chaperone||Pfam PF05051|
|12||Cox18||COX18_HUMAN||Mitochondrial inner membrane protein (Cytochrome c oxidase assembly protein 18)||Pfam PF02096|
|13||Cox19||COX19_HUMAN||Cytochrome c oxidase assembly protein||Pfam PF06747|
|14||Cox20||COX20_HUMAN||Cytochrome c oxidase protein 20 homolog||Pfam PF12597|
- 4 Fe2+-cytochrome c + 8 H+in + O2 → 4 Fe3+-cytochrome c + 2 H2O + 4 H+out
Two electrons are passed from two cytochrome c's, through the CuA and cytochrome a sites to the cytochrome a3- CuB binuclear center, reducing the metals to the Fe2+ form and Cu+. The hydroxide ligand is protonated and lost as water, creating a void between the metals that is filled by O2. The oxygen is rapidly reduced, with two electrons coming from the Fe2+cytochrome a3, which is converted to the ferryl oxo form (Fe4+=O). The oxygen atom close to CuB picks up one electron from Cu+, and a second electron and a proton from the hydroxyl of Tyr(244), which becomes a tyrosyl radical: The second oxygen is converted to a hydroxide ion by picking up two electrons and a proton. A third electron arising from another cytochrome c is passed through the first two electron carriers to the cytochrome a3- CuB binuclear center, and this electron and two protons convert the tyrosyl radical back to Tyr, and the hydroxide bound to CuB2+ to a water molecule. The fourth electron from another cytochrome c flows through CuA and cytochrome a to the cytochrome a3- CuB binuclear center, reducing the Fe4+=O to Fe3+, with the oxygen atom picking up a proton simultaneously, regenerating this oxygen as a hydroxide ion coordinated in the middle of the cytochrome a3- CuB center as it was at the start of this cycle. The net process is that four reduced cytochrome c's are used, along with 4 protons, to reduce O2 to two water molecules.
COX exists in three conformational states: fully oxidized (pulsed), partially reduced, and fully reduced. Each inhibitor has a high affinity to a different state. In the pulsed state, both the heme a3 and the CuB nuclear centers are oxidized; this is the conformation of the enzyme that has the highest activity. A two-electron reduction initiates a conformational change that allows oxygen to bind at the active site to the partially-reduced enzyme. Four electrons bind to COX to fully reduce the enzyme. Its fully reduced state, which consists of a reduced Fe2+ at the cytochrome a3 heme group and a reduced CuB+ binuclear center, is considered the inactive or resting state of the enzyme.
Cyanide, azide, and carbon monoxide all bind to cytochrome c oxidase, thus competitively inhibiting the protein from functioning by preventing the binding of oxygen at the active site, which results in the chemical asphyxiation of cells. Higher concentrations of molecular oxygen are needed to compensate for increasing inhibitor concentrations, leading to an overall reduction in metabolic activity in the cell in the presence of an inhibitor. Other ligands, such as nitric oxide and hydrogen sulfide, can also inhibit COX by binding to regulatory sites on the enzyme, reducing the rate of cellular respiration.
Cyanide is a competitive inhibitor for COX, binding with high affinity to the partially-reduced state of the enzyme and hindering further reduction of the enzyme. In the pulsed state, cyanide binds slowly, but with high affinity. The ligand is posited to electrostatically stabilize both metals at once by positioning itself between them. A high nitric oxide concentration, such as one added exogenously to the enzyme, reverses cyanide inhibition of COX.
Nitric oxide can reversibly bind to either metal ion in the binuclear center to be oxidized to nitrite. NO and CN will compete with oxygen to bind at the site, reducing the rate of cellular respiration. Endogenous NO, however, which is produced at lower levels, augments CN inhibition. Higher levels of NO, which correlate with the existence of more enzyme in the reduced state, lead to a greater inhibition of cyanide. At these basal concentrations, NO inhibition of Complex IV is known to have beneficial effects, such as increasing oxygen levels in blood vessel tissues. The inability of the enzyme to reduce oxygen to water results in a buildup of oxygen, which can diffuse deeper into surrounding tissues. NO inhibition of Complex IV has a larger effect at lower oxygen concentrations, increasing its utility as a vasodilator in tissues of need.
Hydrogen sulfide will bind COX in a noncompetitive fashion at a regulatory site on the enzyme, similar to carbon monoxide. Sulfide has the highest affinity to either the pulsed or partially reduced states of the enzyme, and is capable of partially reducing the enzyme at the heme a3 center. It is unclear whether endogenous H2S levels are sufficient to inhibit the enzyme. There is no interaction between hydrogen sulfide and the fully reduced conformation of COX.
Methanol in methylated spirits is converted into formic acid, which also inhibits the same oxidase system. High levels of ATP can allosterically inhibit cytochrome c oxidase, binding from within the mitochondrial matrix.
Subcellular localization and presence at extramitochondrial sites
Cytochrome c oxidase has 3 subunits which are encoded by mitochondrial DNA. Of these 3 subunits encoded by mitochondrial DNA, two have been identified in extramitochondrial locations. In pancreatic acinar tissue, these subunits were found in zymogen granules. Additionally, in the anterior pituitary, relatively high amounts of these subunits were found in growth hormone secretory granules. The extramitochondrial function of these cytochrome c oxidase subunits has not yet been characterized. Besides cytochrome c oxidase subunits, extramitochondrial localization has also been observed for large numbers of other mitochondrial proteins. This raises the possibility about existence of yet unidentified specific mechanisms for protein translocation from mitochondria to other cellular destinations.
Genetic defects and disorders
Defects involving genetic mutations altering cytochrome c oxidase (COX) functionality or structure can result in severe, often fatal metabolic disorders. Such disorders usually manifest in early childhood and affect predominantly tissues with high energy demands (brain, heart, muscle). Among the many classified mitochondrial diseases, those involving dysfunctional COX assembly are thought to be the most severe.
The vast majority of COX disorders are linked to mutations in nuclear-encoded proteins referred to as assembly factors, or assembly proteins. These assembly factors contribute to COX structure and functionality, and are involved in several essential processes, including transcription and translation of mitochondrion-encoded subunits, processing of preproteins and membrane insertion, and cofactor biosynthesis and incorporation.
Currently, mutations have been identified in seven COX assembly factors: SURF1, SCO1, SCO2, COX10, COX15, COX20, COA5 and LRPPRC. Mutations in these proteins can result in altered functionality of sub-complex assembly, copper transport, or translational regulation. Each gene mutation is associated with the etiology of a specific disease, with some having implications in multiple disorders. Disorders involving dysfunctional COX assembly via gene mutations include Leigh syndrome, cardiomyopathy, leukodystrophy, anemia, and sensorineural deafness.
The increased reliance of neurons on oxidative phosphorylation for energy. facilitates the use of COX histochemistry in mapping regional brain metabolism in animals, since it establishes a direct and positive correlation between enzyme activity and neuronal activity. This can be seen in the correlation between COX enzyme amount and activity, which indicates the regulation of COX at the level of gene expression. COX distribution is inconsistent across different regions of the animal brain, but its pattern of its distribution is consistent across animals. This pattern has been observed in the monkey, mouse, and calf brain. One isozyme of COX has been consistently detected in histochemical analysis of the brain.
Such brain mapping has been accomplished in spontaneous mutant mice with cerebellar disease such as reeler and a transgenic model of Alzheimer's disease. This technique has also been used to map learning activity in animal brain.
- Cytochrome c oxidase subunit I
- Cytochrome c oxidase subunit II
- Cytochrome c oxidase subunit III
- Heme a
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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.
Cytochrome oxidase assembly protein Provide feedback
Mueller JP, Taber HW; , J Bacteriol 1989;171:4967-4978.: Isolation and sequence of ctaA, a gene required for cytochrome aa3 biosynthesis and sporulation in Bacillus subtilis. PUBMED:2549006 EPMC:2549006
Internal database links
This tab holds annotation information from the InterPro database.
InterPro entry IPR003780
This is a family of integral membrane proteins. CtaA (also known as heme A synthase) is required for cytochrome aa3 oxidase assembly in Bacillus subtilis [PUBMED:2549006]. COX15 is required for cytochrome c oxidase assembly in yeast [PUBMED:9228094], and may also be involved in the synthesis of heme A [PUBMED:12474143].In some cases, heme A synthase is found fused to protoheme IX farnesyltransferase, an enzyme that converts protoheme IX to heme O.
The mapping between Pfam and Gene Ontology is provided by InterPro. If you use this data please cite InterPro.
|Cellular component||integral component of membrane (GO:0016021)|
|Molecular function||oxidoreductase activity, acting on the CH-CH group of donors (GO:0016627)|
|Biological process||heme a biosynthetic process (GO:0006784)|
|oxidation-reduction process (GO:0055114)|
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This superfamily includes a variety of different heme binding cytochromes.
The clan contains the following 9 members:COX15-CtaA Cytochrom_B561 Cytochrom_B_N_2 Cytochrom_C_asm Cytochrome_B DUF2427 DUF4405 Ferric_reduct Ni_hydr_CYTB
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- a Java applet developed at the University of Dundee. You will need Java installed before running jalview
- an HTML page showing the whole alignment.Please note: full Pfam alignments can be very large. These HTML views are extremely large and often cause problems for browsers. Please use either jalview or the Pfam viewer if you have trouble viewing the HTML version
- an HTML-based representation of the alignment, coloured according to the posterior-probability (PP) values from the HMM. As for the standard HTML view, heatmap alignments can also be very large and slow to render.
You can download (or view in your browser) a text representation of a Pfam alignment in various formats:
You can also change the order in which sequences are listed in the alignment, change how insertions are represented, alter the characters that are used to represent gaps in sequences and, finally, choose whether to download the alignment or to view it in your browser directly.
You may find that large alignments cause problems for the viewers and the reformatting tool, so we also provide all alignments in Stockholm format. You can download either the plain text alignment, or a gzipped version of it.
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.
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...
If you find these logos useful in your own work, please consider citing the following article:
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.
|Author:||Mian N, Bateman A|
|Number in seed:||64|
|Number in full:||3471|
|Average length of the domain:||261.60 aa|
|Average identity of full alignment:||24 %|
|Average coverage of the sequence by the domain:||80.38 %|
|HMM build commands:||
build method: hmmbuild -o /dev/null HMM SEED
search method: hmmsearch -Z 26740544 -E 1000 --cpu 4 HMM pfamseq
|Family (HMM) version:||14|
|Download:||download the raw HMM for this family|
Weight segments by...
Change the size of the sunburst
selected sequences to HMM
a FASTA-format file
- 0 sequences
- 0 species
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
- save a plain text representation of the tree
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.