In this recipe we'll explore some of the functionality that can be applied to an alignment of protein sequences. We're going to start with a multiple sequence alignment (MSA) of myoglobin sequences from 42 diverse tetrapods, and one human hemoglobin sequence. This multiple sequence alignment was generated by following the steps in the Progressive Multiple Sequence Alignment recipe.
Myoglobin was the first protein sequence to have its stucture determined by x-ray crystallography. For some background on this molecule, see the myoglobin article in the PDB Molecule of the Month series.
First, we'll prepare our environment and load our myoglobin alignment into a
TabularMSA object, setting the MSA's index labels to the sequence identifiers read in from the FASTA file.
%matplotlib inline from skbio import TabularMSA, Protein msa = TabularMSA.read('data/globin-aln.fasta', constructor=Protein) msa.reassign_index(minter='id')
We can next get some information about this alignment. For example, we can easily see that there are 43 sequences in this alignment, and that it's 154 characters long. By definition, there is no variance in sequence length in a multiple sequence alignment. For this reason, it's common to think of an alignment as a matrix or table, where rows represent sequences and columns represent positions in the sequences.
TabularMSA[Protein] ----------------------------------------------------------------------- Stats: sequence count: 43 position count: 154 ----------------------------------------------------------------------- VHLTPEEKSAVTALWGKVNVDEV--GGEALGRL ... KEFTPPVQAAYQKVVAGVANALAHKYH------ -GLSDGEWQLVLKVWGKVEGDLPGHGQEVLIRL ... ADFGADAQAAMGKALELFRNDMAAKYKEFGFQG ... -GLSDDEWNHVLGIWAKVEPDLSAHGQEVIIRL ... SDFGADSQAAMKKALELFRNDMASKYKEFGFQG -GLSDDEWHHVLGIWAKVEPDLSAHGQEVIIRL ... SDFGADSQAAMRKALELFRNDMASKYKEFGFQG
There are also some mechanisms in place to get more direct access to this information. For example:
The index in this alignment (which was set to the sequence identifiers above) contains the genus and species that each sequence is derived from:
for label in msa.index: print(label)
hemoglobin-human Ornithorhynchus.anatinus-platypus Tachyglossus.aculeatus-short.beaked.echidna Macropus.rufus-red.kangaroo Castor.fiber-eurasian.beaver Ondatra.zibethicus-muskrat Kogia.breviceps-pygmy.sperm.whale Physeter.catodon-sperm.whale Ziphius.cavirostris-beaked.whale Orcinus.orca-killer.whale Phocoenoides.dalli.dalli-porpoise Balaenoptera.physalus-fin-whale Inia.geoffrensis-amazon.river.dolphin Canis.lupus.familiaris.dog Bos.taurus-cattle Ovis.aries-sheep Elephas.maximus-asian.elephant Equus.caballus-horse Mus.musculus-mouse Zalophus.californianus-california.sea.lion Orycteropus.afer-aardvark Ctenodactylus.gundi-common.gundi Tupaia.chinensis-treeshrew Lagostomus.maximus-viscacha Proechimys.guairae-guaira.spiny.rat Ochotona.princeps-american.pika Nannospalax.ehrenbergi-blind.mole.rat Lutra.lutra-otter Meles.meles-badger Didelphis.virginiana-opossum Callithrix.jacchus-marmoset Lagothrix.lagotricha-brown.woolly.monkey Saimiri.sciureus-common.squirrel.monkey Rousettus.aegyptiacus-egyptian.fruit.bat Pan.troglodytes-chimpanzee Papio.anubis-baboon Pongo.abelii-orangutan Varanus.varius-monitor Aptenodytes.forsteri-emperor.penguin Alligator.mississippiensis-alligator Gallus.gallus-chicken Caretta.caretta-sea.turtle Graptemys.geographica-map.turtle
We can also explore some more interesting features of this alignment. For example, we can compute conservation for each position in the alignment using the inverse Shannon uncertainty metric. In this case, we're going to treat gap characters as another character in the amino acid alphabet, though the right way to handle gaps in alignments when computing conservation is debatable. To learn more about positional conservation, see
positional_conservation = msa.conservation(metric='inverse_shannon_uncertainty', degenerate_mode='nan', gap_mode='include')
We get an array of positional conservation values (here we'll just print the first ten). A low conservation value means that there is a lot of variation in the sequences at the corresponding position in the alignment, while a high conservation value means that the corresponding position in the alignment is highly conserved. A conservation of 1.0 means that a position is perfectly conserved. A conservation of 0.0 means that every character in the alphabet is present in exactly equal frequency at that position in the alignment (this is uncommon in practice).
for value in positional_conservation[:10]: print(value)
0.963720731598 0.845643534756 1.0 0.902301609936 0.862825292185 0.717505677906 1.0 0.963720731598 0.77120683716 0.730788546142
In our alignment, there are exactly 21 positions that are perfectly conserved across all sequences. This is pretty astonishing, particularly because there is a hemoglobin sequence in the alignment. This means that (based on the information we have here) these positions have not changed since the last common ancestor of the hemoglobin/myoglobin paralogs).
(positional_conservation == 1.0).sum()
Let's figure out what amino acids are at these perfectly conserved positions:
for pos_idx, conservation in enumerate(positional_conservation): if conservation == 1.0: # find the amino acid at this position in the first sequence in the alignment aa = msa[pos_idx] print("Position %3d is perfectly conserved, containing %s in all sequences." % (pos_idx, aa))
Position 2 is perfectly conserved, containing L in all sequences. Position 6 is perfectly conserved, containing E in all sequences. Position 10 is perfectly conserved, containing V in all sequences. Position 14 is perfectly conserved, containing W in all sequences. Position 16 is perfectly conserved, containing K in all sequences. Position 17 is perfectly conserved, containing V in all sequences. Position 25 is perfectly conserved, containing G in all sequences. Position 37 is perfectly conserved, containing P in all sequences. Position 39 is perfectly conserved, containing T in all sequences. Position 43 is perfectly conserved, containing F in all sequences. Position 46 is perfectly conserved, containing F in all sequences. Position 62 is perfectly conserved, containing K in all sequences. Position 65 is perfectly conserved, containing G in all sequences. Position 68 is perfectly conserved, containing V in all sequences. Position 76 is perfectly conserved, containing L in all sequences. Position 89 is perfectly conserved, containing L in all sequences. Position 93 is perfectly conserved, containing H in all sequences. Position 123 is perfectly conserved, containing F in all sequences. Position 130 is perfectly conserved, containing A in all sequences. Position 143 is perfectly conserved, containing A in all sequences. Position 146 is perfectly conserved, containing Y in all sequences.
Finally we can visualize the conservation along the full length of the alignment to get an idea of where the more and less conserved positions are found. In general, positions that are highly conserved are thought to have specific structural or functional roles in the protein, while positions that are less conserved might have less specific roles (e.g., providing a filler between two functional domains in the linear protein backbone).
from pylab import figure fig = figure() ax = fig.add_subplot(111) _ = ax.plot(range(len(positional_conservation)), positional_conservation, '-')
A common reason for building a multiple sequence alignment is for making an inference about the evolutionary relationships between the organisms. We'll do this here using the neighbor joining (
skbio.tree.nj) function in scikit-bio.
The input to
nj is a distance matrix, representing the pairwise distances between the sequences in the alignment. This can be obtained using
DistanceMatrix.from_iterable. Here we'll compute the Hamming distances between all pairs of sequences in the alignment:
from skbio import DistanceMatrix from skbio.sequence.distance import hamming dm = DistanceMatrix.from_iterable(msa, metric=hamming, keys=msa.index) print(dm)
43x43 distance matrix IDs: 'hemoglobin-human', 'Ornithorhynchus.anatinus-platypus', ... Data: [[ 0. 0.73376623 0.74025974 ..., 0.71428571 0.70779221 0.70779221] [ 0.73376623 0. 0.05844156 ..., 0.24025974 0.30519481 0.2987013 ] [ 0.74025974 0.05844156 0. ..., 0.24675325 0.31818182 0.31168831] ..., [ 0.71428571 0.24025974 0.24675325 ..., 0. 0.24025974 0.25324675] [ 0.70779221 0.30519481 0.31818182 ..., 0.24025974 0. 0.05194805] [ 0.70779221 0.2987013 0.31168831 ..., 0.25324675 0.05194805 0. ]]
One initial observation that we can make here is that the mean of the distribution of distances between the hemoglobin sequence and all other sequences is higher than the mean of the distribution of the chimpanzee myoglobin sequences to all of the others. This is exactly what we expect since the hemoglobin sequence diverged from the myoglobin sequences longer ago.
fig = figure() ax = fig.add_subplot(111) ax.hist([dm['hemoglobin-human'], dm['Pan.troglodytes-chimpanzee']], label=['human hemoglobin', 'chimpanzee myoglobin']) ax.set_xlim((0,1)) ax.legend(loc='upper left')
<matplotlib.legend.Legend at 0x102023cc0>
We can now build a neighbor-joining tree from this distance matrix:
from skbio.tree import nj tree = nj(dm)
import ete3 ts = ete3.TreeStyle() ts.show_branch_length = True ete3.TreeNode.from_skbio(tree).render("%%inline", tree_style=ts)
To get a better understanding of the evolutionary history, we can use outgroup rooting to place a root in the tree. When doing outgroup rooting, you want to find the tip corresponding to the most distantly related sequence in the alignment, and look at its most recent ancestral node. Because we know a priori that the hemoglobin sequence is the most distantly related of the sequences in our alignment (because it's not myoglobin, so by definition had to have begun diverging before all of our myoglobin sequences began diverging), you know that its first ancestral node should be the common ancestor of all sequences in the tree.
First we find the node that we want to be the new root.
new_root = tree.find('hemoglobin-human').ancestors()
And then we build a new tree rooted at this node:
rooted_tree = tree.root_at(new_root)
Visualizing the tree with ETE, you can see that this actually gives us a pretty good picture of the relationships between the organisms. For example, notice that the deepest splits are the myoglobin versus hemoglobin clades, then the separation of the reptiles, birds, and egg-laying mammals, respectively.
ts = ete3.TreeStyle() ts.show_branch_length = True ete3.TreeNode.from_skbio(rooted_tree).render("%%inline", tree_style=ts)