How do I get a reasonably complete, reasonably accurate list of PPIs?



A recurring thing we are interested in is classifying the natural ligand/partner of a protein. Having a classification of the 'natural' ligand partner for every human protein would be really useful, and would allow us to ask and more importantly answer quite complex questions in ChEMBL like - 'show me all small molecules that modulate a protein-protein interaction (PPI) at better than 100 nM potency?'.

I've had a bit of a dig around the web, but there isn't anything that does exactly what I need (or I'm not smart enough to find it), but I'm sure it must exist, so in the comments section, please post resources and ideas.....