Phosphorylation-dependent and -independent nuclear import of RS domain-containing splicing factors and regulators.

SR proteins and related RS domain-containing polypeptides are an important class of splicing regulators in higher eukaryotic cells. The RS domain facilitates nuclear import of SR proteins and mediates protein-protein interactions during spliceosome assembly; both functions appear to subject to regulation by phosphorylation. Previous studies have identified two nuclear import receptors for SR proteins, transportin-SR1 and transportin-SR2. Here we show that transportin-SR1 and transportin-SR2 are the alternatively spliced products of the same gene and that transportin-SR2 is the predominant transcript in most cells and tissues examined. While both receptors import typical SR proteins in a phosphorylation-dependent manner, they differentially import the RS domain-containing splicing regulators hTra2alpha and hTra2beta in different phosphorylation states. We suggest that differential regulation of nuclear import may serve as a mechanism for homeostasis of RS domain-containing splicing factors and regulators in the nucleus and for selective cellular responses to signaling.