Component of the U1 snRNP particle, which recognizes and binds the 5'-splice site of pre-mRNA. Together with other non-snRNP factors, U1 snRNP forms the spliceosomal commitment complex, that targets pre-mRNA to the splicing pathway. Component of the U1 snRNP particle, a subcomplex of the spliceosome. Belongs to the Luc7 family.