Regulated cellular partitioning of SR protein-specific kinases in mammalian cells.
Reversible phosphorylation of the SR family of splicing factors plays an important role in pre-mRNA processing in the nucleus. Interestingly, the SRPK family of kinases specific for SR proteins is localized in the cytoplasm, which is critical for nuclear import of SR proteins in a phosphorylation-dependent manner. Here, we report molecular dissection of the mechanism involved in partitioning SRPKs in the cytoplasm. Common among all SRPKs, the bipartite kinase catalytic core is separated by a unique spacer sequence. The spacers in mammalian SRPK1 and SRPK2 share little sequence homology, but they function interchangeably in restricting the kinases in the cytoplasm. Removal of the spacer in SRPK1 had little effect on the kinase activity, but it caused a quantitative translocation of the kinase to the nucleus and consequently induced aggregation of splicing factors in the nucleus. Rather than carrying a nuclear export signal as suggested previously, we found multiple redundant signals in the spacer that act together to anchor the kinase in the cytoplasm. Interestingly, a cell cycle signal induced nuclear translocation of the kinase at the G2/M boundary. These findings suggest that SRPKs may play an important role in linking signaling to RNA metabolism in higher eukaryotic cells.
Ding
Jian-Hua
JH
Department of Cellular and Molecular Medicine, University of California, San Diego, La Jolla, CA 92093-0651, USA.
Zhong
Xiang-Yang
XY
Hagopian
Jonathan C
JC
Cruz
Marissa M
MM
Ghosh
Gourisankar
G
Feramisco
James
J
Adams
Joseph A
JA
Fu
Xiang-Dong
XD
D021581:Active Transport, Cell Nucleus; D000818:Animals; D002467:Cell Nucleus / Q000201:enzymology*; D006367:HeLa Cells; D006801:Humans; D007399:Interphase; D007700:Kinetics; D009687:Nuclear Proteins / Q000378:metabolism; D010766:Phosphorylation; D011499:Protein Processing, Post-Translational; D017346:Protein Serine-Threonine Kinases / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D021382:Protein Sorting Signals; D012313:RNA / Q000378:metabolism; D012326:RNA Splicing / Q000502:physiology; D016601:RNA-Binding Proteins; D000068103:Serine-Arginine Splicing Factors; D015398:Signal Transduction
D009687:Nuclear Proteins; D021382:Protein Sorting Signals; D016601:RNA-Binding Proteins; D000068103:Serine-Arginine Splicing Factors; D012313:RNA; C087728:SRPK1 protein, human; D017346:Protein Serine-Threonine Kinases; C110522:SRPK2 protein, human