Involved in nuclear mRNA splicing. The principal role of the U1A is to help fold or maintain U1 RNA in an active configuration. It 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. Component of the spliceosome where it is associated with snRNP U1. Present with 2950 molecules/cell in log phase SD medium. Belongs to the RRM U1 A/B'' family.