Hello all, I have a question regarding co-crystals with peptides, please do share any experiences you may have had. In my group we are trying to crystalise 15KDa protein with a ~1KDa peptide (related to a binding partner). We initially set up apo crystallisation and protein:peptide at a 1:1 molar ratio. We had crystals in both with very similar morphology (plates), the protein:peptide mix crystals diffracted well and gave us a good structure, but no peptide was bound, however the purely apo tray gave crystals with much poorer anomalous diffraction. It looks therefore like the peptide is helping the protein to crystalise in a more ordered form, but maybe the concentration isn't high enough to reach saturation. ITC indicates that this peptide does bind to our protein.
We have set up further crystallisation with a protein:peptide ratio of 1:5 and have again grown crystals, however the morphology and plate profiles look exactly the same as before. In my experience with compounds this is quite common (and of course plates are a common morphology) but would you expect with the larger peptides to see this? Or would you expect something different? If not in morphology in the cell dimensions of the crystal? Is it worth collecting a dataset for these crystals to see if the peptide is bound?
Some more information is that the crystallisation is very fast (concentration dependant), within minutes to hours. The peptide is also co-purified with the protein.
Thanks in advance for any help.
|