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. SNRPC/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. 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 3 U1 snRNP-specific proteins SNRNP70/U1-70K, SNRPA/U1-A and SNRPC/U1-C. SNRPC/U1-C interacts with U1 snRNA and the 5' splice-site region of the pre-mRNA (By similarity). Interacts (via N-terminus) with TIA1 (via C-terminus); thereby promoting spliceosomal U1 snRNP recruitment to 5' splice sites (By similarity). Widely expressed. In the testis, expressed in somatic and germinal testicular cells but not in elongated spermatids. First detected in the testis 18 days after birth. Levels increase successively between days 21-28. On day 42, levels decrease. Belongs to the U1 small nuclear ribonucleoprotein C family.