Orthologs from crop in model plants and vice versa
Let’s imagine you are a beginner in the field of plant science, and glutamine synthetase (GS) is the focus of your interest. You have been dealing with the literature and you have come across a paper that awakened your interest: “Atomic Structure of Plant Glutamine Synthetase. A Key Enzyme for Plant Productivity” (J. Biol. Chem 29: 29287-29296).
In this paper, the authors report the crystal structure of maize GS. However, as the authors acknowledge, higher plants have several isoenzymes of GS differing in heat stability and catalytic properties. The authors refer to the isoenzyme they are characterizing as GS1a, being Ile-161 a key residue responsible for the heat stability of this protein. Your aim is to find, if they exist, orthologs of this maize protein in the model plant Arabidopsis thaliana. You may be a beginner, but you are aware that both maize and arabidopsis genomes have been shown to contain six GS genes each.
At first you’re slightly bewildered: none of the corn isoforms is called GS1a! But, everyone knows that GS1 implies a cytosolic form of the enzyme (while GS2 refers to the chloroplastidial one), so our protein should be one of the five cytosolic isoenzymes present in maize. Taking into consideration the fact that isoleucine should be present at position 161 and other information related to the sequence that is provided in the above mentioned paper, we conclude that GS1a matches the GS1-4 from UniProt (or Zm_GS1b_4 using the phylo identifier of orthGS).
As a side note, we would like to point that the choice of GS1a to name this maize isoform was somewhat unfortunate. Indeed, among researcher in the gymnosperm field, GS1a is a term used to describe a set of evolutionary close cytosolic proteins whose expression and function is related to photosynthesis and photorespiration, reminiscent of GS2 in angiosperms.
In any case, we have already identified in maize the protein of interest (Zm_GS1b_4). Now, we have to look for orthologs in arabidopsis. Of course, the quickest and easiest way to do that is with the R package orthGS, but let’s pretend for a moment that we don’t know this resource. Thus, we are going to download all the arabidopsis and maize sequences, align them and build a phylogenetic tree, to see if this approach suggests something to us.
The function subsetGS() takes as an argument the species
of interest and returns a dataframe with the sequences and information
regarding the GS isoforms found in these species.
Afterwards, we can proceed with the alignment and phylogenetic tree
construction. For the first of these tasks, that is, the alignment, you
can use whatever your favorite resource is. Nevertheless, the function
msa of orthGS allows you to perform the
task fluently. Note, that to use this function you need the executable
of either MUSCLE or Clustal-Omega in the PATH of your system (see the
vignette Performing Sequence Alignment in R).
# aln <- msa(sequences = maize_ara$prot, ids = maize_ara$phylo_id)
# a <- aln$ali
# rownames(a) <- maize_ara$phylo_id
# tr <- mltree(a)$tree
# plot(phangorn::midpoint(tr), cex = 0.7)If we want to be rigorous, and of course we do, there is little, if
anything, to conclude about orthology relationships between maize and
arabidopsis GS proteins in view of these results. So, it’s time to
switch to a more suitable approach. Firstly, we will use the function
orthG(), which takes as an argument the set of species to
be included in the analysis and returns an orthology graph: two nodes
(two GS proteins) are connected if and only if they are orthologs (an
adjacency matrix is also provided if wished).
Let’s now include rice in the set of species to be analyzed:
As you can notice, the enzyme Osa_GS1b_1 is orthologous to the maize Zm_GS1b_4. Furthermore, rice and maize share a few orthologs, but arabidopsis has no orthologs neither in rice nor maize.
A function slightly different from the one we have just used, which
you might find useful on occasion is orthP(). Let’s see it
in action:
In red, the GS protein whose ortholgs we are searching for are presented. In blue, the proteins detected as ortholgs in other plant species.
While Zm_GS1b_4 is orthologous to any GS1b from gymnosperms, among the angiosperms we only find three orthologs: Atr_GS1b_2, Sly_GS1b2 and Osa_GS1b_1. To decode which species are Atr, Sly and Osa, we proceed as follows: