A novel Physarum polycephalum SR protein kinase specifically phosphorylates the RS domain of the human SR protein, ASF/SF2.

A 1591-bp cDNA of a serine-rich protein kinase (SRPK)-like protein has been identified in Physarum polycephalum (GenBank accession No. DQ140379). The cDNA contains two repeat sequences at bp 1-153 and bp 395-547. The encoding sequence is 56% homologous to human SRPK1 and is named Physarum SRPK (PSRPK). Consistent with other SRPKs, the consensus motifs of PSRPK are within the two conserved domains (CDs). However, divergent motifs between the N-terminal and CDs are much shorter than the corresponding sequences of other SRPKs. To study the structure and function of this protein, we performed co-expression experiment in Escherichia coli and in vitro phosphorylation assay to investigate the phosphorylation effect of recombinant PSRPK on the human SR protein, ASF/SF2. Western blot analysis showed that PSRPK could phosphorylate ASF/SF2 in E. coli cells. Autoradiographic examination showed that both recombinant PSRPK and a truncated form of PSRPK with a 28-aa deletion at the N-terminus could phosphorylate ASF/SF2 and a truncated form of ASF/SF2 that contains the RS domain. However, these two forms of PSRPK could not phosphorylate a truncated form ASF/SF2 that lacks the RS domain. A truncated form of PSRPK that lacks either of CDs does not have any phosphorylation activity. These results indicated that, like other SRPKs, the phosphorylation site in PSRPK is located within the RS domain of the SR protein and that its phosphorylation activity is closely associated with the two CDs. This study on the structure and function of PSRPK demonstrates that it is a new member of the SRPK family.