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. usp103/U1-C is directly involved in initial 5' splice-site recognition for both constitutive and regulated alternative splicing. The interaction with the 5' splice-site seems to precede base-pairing between the pre-mRNA and the U1 snRNA. Stimulates commitment or early (E) complex formation by stabilizing the base pairing of the 5' end of the U1 snRNA and the 5' splice-site region. U1 snRNP is composed of the 7 core Sm proteins smb1, smd1, smd2, smd3, sme1, smf1 and smg1 (Sm proteins B, D1, D2, D3, E, F and G, respectively) that assemble in a heptameric protein ring on the Sm site of the small nuclear RNA to form the core snRNP, and at least 9 U1 snRNP-specific proteins usp101/U1-70K, usp102/U1-A, usp103/U1-C, usp106/LUC7, usp105/PRP39, usp104/PRP40, usp107/U1-H, usp108/U1-J and usp109/U1-L. usp103/U1-C interacts with U1 snRNA and the 5' splice-site region of the pre-mRNA. Belongs to the U1 small nuclear ribonucleoprotein C family.