1. Perform a search by putting the UniProtKB ids "PSB5_HUMAN" and "PSA7_HUMAN" in the search box.
2. Once you have your table of results (88 interactions as of March 2012), you can remove the 26 N-ary interactions, where you are not sure whether the proteins are interacting as a binary pair or not, by clicking the "filter" link.
3. Now you end up with just 62 interactions. you can search the table to see whether the two proteasome subunits are interacting or if they share common interacting partners. However, it will be easier if you represent the data as a graphical network.
4. Click on the "Graph" tab.
5. In the graphical representation you can see that PSMA7 and PSMB5 are not directly interacting, but that there are three links between them through PSMA3, EPB41 and RIPK2.