Transportin-SR, a nuclear import receptor for SR proteins

The SR proteins, a group of abundant arginine/serine (RS)-rich proteins, ar e essential pre-mRNA splicing factors that are localized in the nucleus,The RS domain of these proteins serves as a nuclear localization signal. We fo und that RS domain-bearing proteins do not utilize any of the known nuclear import receptors and identified a novel nuclear import receptor specific f or SR proteins. The SR protein import receptor. termed transportin-SR (TRN- SR), binds specifically and directly to the RS domains of ASF/SF2 and SC35 as well as several other SR proteins. The nuclear transport regulator RanGT P abolishes this interaction. Recombinant TRN-SR mediates nuclear import of RS domain-bearing proteins in vitro. TRN-SR has amino acid sequence simila rity to several members of the importin beta/transportin family. These find ings strongly suggest that TRN-SR is a nuclear import receptor for the SR p rotein family.