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. SNRNP70 binds to the loop I region of U1-snRNA. Component of the U1 snRNP. 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 (By similarity). Interacts with SCNM1 (By similarity). Found in a pre-mRNA splicing complex with SFRS4, SFRS5, SNRNP70, SNRPA1, SRRM1 and SRRM2. Found in a pre-mRNA exonic splicing enhancer (ESE) complex with SNRNP70, SNRPA1, SRRM1 and TRA2B/SFRS10. Interacts with dephosphorylated SFRS13A and SFPQ. Interacts with NUDT21/CPSF5, CPSF6, SCAF11, and ZRANB2. Interacts with GEMIN5 (By similarity). Interacts with FUS (By similarity). Colocalizes with SCNM1 and LUC7L2 in nuclear speckles. The RRM domain mediates interaction with U1 RNA. Extensively phosphorylated on serine residues in the C-terminal region.