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Background  Serine/arginine (SR) protein-specific kinases (SRPKs) are conserved in a wide range of organisms, from humans to yeast. Studies showed that SRPKs can regulate the nuclear import of SR proteins in cytoplasm, and regulate the sub-localization of SR proteins in the nucleus. But no nuclear localization signal (NLS) of SRPKs was found. We isolated an SRPK-like protein PSRPK (GenBank accession No.  DQ140379 ) from  Physarum polycephalum  previously, and identified a NLS of PSRPK in this study.    Results  We carried out a thorough molecular dissection of the different domains of the PSRPK protein involved in its nuclear localization. By truncation of PSRPK protein, deletion of and single amino acid substitution in a putative NLS and transfection of mammalian cells, we observed the distribution of PSRPK fluorescent fusion protein in mammalian cells using confocal microscopy and found that the protein was mainly accumulated in the nucleus; this indicated that the motif contained a nuclear localization signal (NLS). Further investigation with truncated PSPRK peptides showed that the NLS ( 318 PKKGDKYDKTD 328 ) was localized in the alkaline Ω-loop of a helix-loop-helix motif (HLHM) of the C-terminal conserved domain. If the  318 PKKGDK 322  sequence was deleted from the loop or K 320  was mutated to T 320 , the PSRPK fluorescent fusion protein could not enter and accumulate in the nucleus.    Conclusion  This study demonstrated that the  318 PKKGDKYDKTD 328  peptides localized in the C-terminal conserved domain of PSRPK with the Ω-loop structure could play a crucial role in the NLS function of PSRPK.

Identification of a nuclear localization motif in the serine/arginine protein kinase PSRPK of physarum polycephalum