Pre-mRNA splicing is a key process in the regulation of gene expression as reflected by its disruption in various diseases. Previously, we have demonstrated that the spliceosome-associated factor Nrl1 of the fission yeast S. pombe, except of its role in suppression of accumulation of genome-threatening R-loops, also affects splicing and expression of several genes and non-coding RNAs. To uncover the molecular mechanisms regulating the function of Nrl1, we performed here the systematic analysis of its protein-protein interactions at the domain level. We found that N-terminal region of Nrl1 secures the interaction of Nrl1 with ATP-dependent RNA helicase Mtl1 but is incapable of docking of Nrl1 into the spliceosome. Contrary, the C-terminal region of Nrl1 was found to be critical for the interactions of Nrl1 with other splicing factors and the stable binding of Nrl1 into the spliceosome. Importantly, we showed that splicing function of Nrl1 depends on its phosphorylation. Additionally, we identified two amino acid residues of Nrl1, namely S122 and S131, which are directly phosphorylated by Cka1 protein kinase. Together, our results indicate the novel features involved in the regulation of spliceosome-associated factor Nrl1, defined by its domain-dependent interactions and by the CKII-dependent phosphorylation.