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Mol Cell Biol, February 1998, p. 676-684, Vol. 18, No. 2
0270-7306/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.

Sip1, a Novel RS Domain-Containing Protein Essential for Pre-mRNA Splicing

Wan-Jiang Zhang and Jane Y. Wu*

Department of Pediatrics and Department of Molecular Biology and Pharmacology, Washington University School of Medicine, St. Louis, Missouri 63110

Received 1 July 1997/Returned for modification 19 August 1997/Accepted 6 November 1997

Previous studies have shown that protein-protein interactions among splicing factors may play an important role in pre-mRNA splicing. We report here identification and functional characterization of a new splicing factor, Sip1 (SC35-interacting protein 1). Sip1 was initially identified by virtue of its interaction with SC35, a splicing factor of the SR family. Sip1 interacts with not only several SR proteins but also with U1-70K and U2AF65, proteins associated with 5' and 3' splice sites, respectively. The predicted Sip1 sequence contains an arginine-serine-rich (RS) domain but does not have any known RNA-binding motifs, indicating that it is not a member of the SR family. Sip1 also contains a region with weak sequence similarity to the Drosophila splicing regulator suppressor of white apricot (SWAP). An essential role for Sip1 in pre-mRNA splicing was suggested by the observation that anti-Sip1 antibodies depleted splicing activity from HeLa nuclear extract. Purified recombinant Sip1 protein, but not other RS domain-containing proteins such as SC35, ASF/SF2, and U2AF65, restored the splicing activity of the Sip1-immunodepleted extract. Addition of U2AF65 protein further enhanced the splicing reconstitution by the Sip1 protein. Deficiency in the formation of both A and B splicing complexes in the Sip1-depleted nuclear extract indicates an important role of Sip1 in spliceosome assembly. Together, these results demonstrate that Sip1 is a novel RS domain-containing protein required for pre-mRNA splicing and that the functional role of Sip1 in splicing is distinct from those of known RS domain-containing splicing factors.

* Corresponding author. Mailing address: Department of Pediatrics and Department of Molecular Biology and Pharmacology, Washington University School of Medicine, One Children's Place, St. Louis, MO 63110. Phone: (314) 454-2081. Fax: (314) 454-2075. E-mail: WU_J{at}A1.kids.wustl.edu.




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