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. U1 snRNP is the first snRNP to interact with pre-mRNA. This interaction is required for the subsequent binding of U2 snRNP and the U4/U6/U5 tri-snRNP. Snrpa binds stem loop II of U1 snRNA (By similarity). U1 snRNP is composed of 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. Belongs to the RRM U1 A/B'' family.