Reactome is a human-centric database of the roles that proteins play in biological pathways. While IntAct focusses on the interaction of proteins with other target molecules, Reactome looks at proteins associated with biological processes, breaking each process down into a series of molecular events. Reactome events have inputs (substrates) and outputs (products). Like IntAct, Reactome is curated by life scientists, using evidence from the scientific literature and input from experts in each pathway. The curation rules can be found on the Reactome wiki.
The Reactome Pathway Analysis Tool can be used to 'paint' pathways with a list of proteins (e.g. a list of UniProt accessions derived from a proteomics experiment) or genes (e.g. a list of EMBL/GenBank/DDBJ IDs derived from a functional genomics experiment). Reactome will display pathways associated with your submitted data, with links from within Reactome to biological process entries. Reactome uses UniProtKB IDs, which can be followed to a UniProtKB summary page.