I've downloaded the human PPIs and ID mappings from BioGRID and gene ontologies from geneontology.org, and I'd like to combine them into a single dataset. In their respective files, PPIs are identified by the entrez gene of each interactor while ontologies are associated to UnitProtKB IDs. However, in the BioGRID mappings file, some entrez genes are mapped to multiple UniProtKB IDs. Why does this happen? Considering this, what would be the most correct way to combine the two databases?
My background is in computer engineering, not biology or anything related, so I'm pretty confused. Help would be much appreciated :)