First, the snRNP component U1A was HA-tagged and the intron of the RabX13 gene was MS2-tagged and amoeba transformats were established with such constructs. Next, amoeba transformants were UV cross-linked and their nucler extracts were immunoprecipitated (IP) with anti-HA agarose or MS2-spharose beads. Precipitates were subjected to tandem mass spectrometry (MS/MS) analyses. MS2-IP helped to discriminate the nuclear roles (chromatin-, co-transcriptional-, splicing-related), of the proteins probed.