Protein-protein interactions search
You can try to use IntAct Molecular Interaction Database. The data in this database is based on:


data from the literature or from direct data depositions by expert
curators  
&lt;300k binary interactions in 2011  developed by the EBI's
Proteomics Services Team  
recent reference: Aranda et al. '10



This is also mentioned in the summary of the Aranda et al.'10 article:


  IntAct is an open-source, open data molecular interaction database and
  toolkit. Data is abstracted from the literature or from direct data
  depositions by expert curators following a deep annotation model
  providing a high level of detail. As of September 2009, IntAct
  contains over 200.000 curated binary interaction evidences. In
  response to the growing data volume and user requests, IntAct now
  provides a two-tiered view of the interaction data. The search
  interface allows the user to iteratively develop complex queries,
  exploiting the detailed annotation with hierarchical controlled
  vocabularies. Results are provided at any stage in a simplified,
  tabular view. Specialized views then allows 'zooming in' on the full
  annotation of interactions, interactors and their properties. IntAct
  source code and data are freely available


Further you can use the Evidence tab in String(I assume you have already tried this but in the case you did not): 

You can then click on Experiments, then something like this will show up:

You can see how (e.g. Reconstituted Complex assay) and you can see from which article this interaction is coming from if you click on an item in the table: