Phosphoproteomics Reveals Novel Targets and Phosphoprotein Networks in Cell Cycle Mediated by Dsk1 Kinase

As
the ortholog of human SR protein kinase 1 in fission yeast Schizosaccharomyces pombe, Dsk1 specifically phosphorylates
SR proteins (serine/arginine-rich proteins) and promotes splicing
of nonconsensus introns. The SRPK (SR protein-specific kinase) family
performs highly conserved functions in eukaryotic cells including
cell proliferation, differentiation, development, and apoptosis. Although
Dsk1 was originally identified as a mitotic regulator, its specific
targets involved in cell cycle have yet been unexplored. In this study,
using a phosphoproteomics approach, we examined differential protein
phosphorylation between wild-type cells and dsk1-deletion
mutants. We found reduced phosphorylation of 149 peptides corresponding
to 133 proteins in the dsk1-null cells. These proteins
are involved in various cellular processes, including cytoskeleton
organization and signal transduction, and specifically enriched in
multiple steps of cell cycle control. Further, targeted MS analyses
and in vitro biochemical assays established Cdr2 protein kinase and
kinesin motor Klp9 as novel substrates of Dsk1, which function in
cell size control for mitotic entry and in chromosome segregation
for mitotic exit, respectively. The phosphoprotein networks mediated
by Dsk1 reveal, for the first time, the molecular links connecting
Dsk1 to mitotic phase transition, sister-chromatid segregation, and
cytokinesis, providing further evidence of Dsk1’s diverse influence
on cell cycle progression and regulation.