I'll not lie to you. This was on my "to-do" list since December and I didn't think I'd get to it until next December but I lucked into a couple spare hours on Friday and here we go.
Without further ado: how to do N15 quantification in Proteome Discoverer 2.0. This is similar to how my old tutorial went for PD 1.3. (BTW, there is an alternative way of doing this in PD 2.0 that I just figured out, but I think this one will be the cleanest! Lets see!)
Go to Administration > Maintain Chemical Modifications. And add these modifications (click to expand):
If you want to add it to an existing Study you will now need to save that study, close it and re-open it for this method to work. Better yet? Just close any open studies. I'm not done yet.
Go to Quantification methods (still in Administration). Add a new Quantification method. You can write it from scratch OR you can clone it from a different method. Here I'm cloning it from the Full 18O method:
Alright...this is where it gets complicated. Different amino acids can integrate different amounts of N15. Some only have one N, while others have up to 4. You need to create 4 labels, one for each possibility. It ought to look something like this:
Next...well...you need a good dataset to test it on. If you are interested in this topic its likely that you already have one. I...didn't. Fortunately, however, I found a fantastic study from a couple of years ago from H. Zhang et. al., out of Utreckt. If you want direct access to the data it is PRIDE dataset PXD000177 (direct link? not sure if you can direct link into PRIDE due to user account stuff).
Set up your workflow the way you usually would (I used the pre-made SILAC template) but add all of your modifications:
Set up your Consensus as normal. Just make sure the "Peptide and Protein Quantifier" node is involved. If it isn't you should get a little pop-up to remind you, but its better to just do it yourself :)
Note: there is a slightly easier way around this on newer instruments. Since this is an MS1-centric quantification method you don't actually have to have fragmentation of both your light and heavy species to get quantification. In fact, you can set your instrument to only fragment your light species (or heavy) then you don't have to use data intensive dynamic mods for your searching. Just a thought. Here I'm just doing both.
I'm going to have a coffee while Percolator digs through 27 dynamic modifications...actually, it wasn't that bad...and I appear to have packed all the coffee (moving is AWESOME!).
Okay, here is the $64 question: Did it work? Yeah, I think it did...
I ran one SCX fraction from this study. They did extensive SCX fractionation. Only in 2 cases did I have 2 peptides from the same protein identified (hence the missing ratio standar errors). But I generated ratios...and...
...I get quantification spectra that seem to make sense. This peptide was found as a triply charged heavy and light and looks pretty believable. And crude math in my head according to the length of the proteins makes this 24 Da shift between light and heavy seem perfect to the peptide sequence.
I'll try to download one of the full datasets from this study and search the data the same way they did and see if I can match their results. Now...this needs said... This method isn't perfect. There are better ways of doing this experiment (check out the awesome software package written by Princeton and mentioned in an earlier blog post) but PD can do it. And if you really want to do it this way you really can iron out the rest of the bugs. But this ought to help you get started. I'll try to put together a video for this when I have time.