Functions as an U snRNP-specific nuclear import adapter. Involved in the trimethylguanosine (m3G)-cap-dependent nuclear import of U snRNPs. Binds specifically to the terminal m3G-cap U snRNAs. {ECO:0000269|PubMed:10209022, ECO:0000269|PubMed:15920472, ECO:0000269|PubMed:16030253, ECO:0000269|PubMed:38413582, ECO:0000269|PubMed:9670026}. This is the function of ENSG00000169371 (SNUPN, snurportin 1).