Human PRPF39 is an alternative splicing factor recruiting U1 snRNP to weak 5′ splice sites

Human PRPF39 is a homolog of the yeast Prp39 and Prp42 paralogs. We have previously shown that human PRPF39 forms a homodimer that interacts with the CTD of U1C, mirroring the yeast Prp39/Prp42 heterodimer. We demonstrate here that PRPF39 knockdown in HEK293 cells affects many alternative splicing events primarily by reducing the usage of weak 5 ' ss. Additionally, PRPF39 preferentially binds to a GC-rich RNA, likely at the interface between its NTD and CTD. These data indicate that PRPF39 potentially recruits U1 snRNP to a weak 5 ' ss, serving as a previously unrecognized alternative splicing factor. We further demonstrate that human TIA1 binds to U1C through its RRM1 and RRM3 + Q domains and has weak binding to PRPF39. Finally, all three human LUC7L isoforms directly interact with U1C and with PRPF39. These results reveal significant parallels to the yeast U1 snRNP structure and support the use of yeast U1 snRNP as a model for understanding the mechanism of human alternative splicing.