THE MAMMALIAN HOMOLOG OF PRP16P IS OVEREXPRESSED IN A CELL-LINE TOLERANT TO LEFLUNOMIDE, A NEW IMMUNOREGULATORY DRUG EFFECTIVE AGAINST RHEUMATOID-ARTHRITIS

Prp2p, Prp16p, Prp22p, and Prp43p are members of the BEAN-box family o f ATP-dependent putative RNA helicases required for pre-mRNA splicing in Saccharomyces cerevisiae. Recently, mammalian homologues of Prp43p and Prp22p have been described, supporting the idea that splicing in y east and man is phylogenetically conserved. In this study, we show tha t a murine cell line resistant to the novel immunoregulatory drug Lefl unomide (Arava(TM)) overexpresses a 135-kDa protein that is a putative DEAH-box RNA helicase. We have cloned the human counterpart of this p rotein and show that it shares pronounced sequence homology with Prp16 p. Apart from its N-terminal domain, which is rich in RS, RD, and RE d ipeptides, this human homologue of Prp16p (designated hPrp16p) is 41% identical to Prp16p. Significantly, homology is not only observed with in the phylogenetically conserved helicase domain, but also in Prp16p- specific sequences. Immunofluorescence microscopy studies demonstrated that hPrp16p co-localizes with snRNPs in subnuclear structures referr ed to as speckles. Antibodies specific for hPrp16p inhibited pre-mRNA splicing in vitro prior to the second step. Thus, like its yeast count erpart, hPrp16p also appears to be required for the second catalytic s tep of splicing. Taken together, our data indicate that the human 135- kDa protein identified here is the structural and functional homologue of the yeast putative RNA helicase, Prp16p.