Component of the spliceosomal U1 snRNP, which is essential for recognition of the pre-mRNA 5' splice-site and the subsequent assembly of the spliceosome (PubMed:19325628, PubMed:25555158). SNRNP70 binds to the loop I region of U1-snRNA (PubMed:2467746, PubMed:19325628, PubMed:25555158). Truncated isoforms that lack the RRM domain cannot bind U1-snRNA. Truncated isoforms that lack the RRM domain cannot bind U1-snRNA. Component of the U1 snRNP (PubMed:19325628, PubMed:21113136, PubMed:25555158). The U1 snRNP is composed of the U1 snRNA and the 7 core Sm proteins SNRPB, SNRPD1, SNRPD2, SNRPD3, SNRPE, SNRPF and SNRPG that assemble in a heptameric protein ring on the Sm site of the small nuclear RNA to form the core snRNP, and at least three U1 snRNP-specific proteins SNRNP70/U1-70K, SNRPA/U1-A and SNRPC/U1-C (PubMed:19325628, PubMed:21113136, PubMed:25555158). Interacts with SCNM1 (By similarity). Found in a pre-mRNA splicing complex with SFRS4, SFRS5, SNRNP70, SNRPA1, SRRM1 and SRRM2 (PubMed:9531537). Found in a pre-mRNA exonic splicing enhancer (ESE) complex with SNRNP70, SNRPA1, SRRM1 and TRA2B/SFRS10 (PubMed:10339552). Interacts with dephosphorylated SFRS13A and SFPQ (PubMed:11514619, PubMed:14765198). Interacts with NUDT21/CPSF5, CPSF6, SCAF11, and ZRANB2 (PubMed:14561889, PubMed:11448987, PubMed:9447963). Interacts with GEMIN5 (PubMed:25911097). Interacts with FUS. Colocalizes with SCNM1 and LUC7L2 in nuclear speckles. The RRM domain mediates interaction with U1 RNA. The N-terminus is blocked. Extensively phosphorylated on serine residues in the C-terminal region. Major ribonucleoprotein antigen recognized by the sera from patients with autoimmune diseases, such as systemic lupus erythematosus. Extended N-terminus. Extended N-terminus. Extended N-terminus.