A SPECIFIC SUBSET OF SR PROTEINS SHUTTLES CONTINUOUSLY BETWEEN THE NUCLEUS AND THE CYTOPLASM

The SR proteins constitute a large family of nuclear phosphoproteins r equired for constitutive pre-mRNA splicing. These factors also have gl obal, concentration-dependent effects on alternative splicing regulati on and this activity is antagonized by members of the hnRNP A/B family of proteins. We show here that whereas some human SR proteins are con fined to the nucleus, three of them-SF2/ASF, SRp20, and 9G8-shuttle ra pidly and continuously between the nucleus and the cytoplasm. By swapp ing the corresponding domains between shuttling and nonshuttling SR pr oteins, we show that the carboxy-terminal arginine/serine-rich (RS) do main is required for shuttling. This domain, however, is not sufficien t to promote shuttling of an unrelated protein reporter, suggesting th at stable RNA binding mediated by the RNA-recognition motifs may be re quired for shuttling. Consistent with such a requirement, a double poi nt-mutation in RRM1 of SF2/ASF that impairs RNA binding prevents the p rotein from shuttling. In addition, we show that phosphorylation of th e RS domain affects the shuttling properties of SR proteins. These fin dings show that different SR proteins have unique intracellular transp ort properties and suggest that the family members that shuttle may ha ve roles not only in nuclear pre-mRNA splicing but also in mRNA transp ort, cytoplasmic events, and/or processes that involve communication b etween the nucleus and the cytoplasm.