Targeted quantification of phosphorylation sites identifies STRIPAK-dependent phosphorylation of the Hippo pathway-related kinase SmKIN3

We showed recently that the germinal centre kinase III (GCKIII) SmKIN3 from the fungus Sordaria macrospora is involved in sexual development and hyphal septation. Our recent extensive global proteome and phosphoproteome analysis revealed that SmKIN3 is a target of the striatin interacting phosphatase and kinase (STRIPAK) multi-subunit complex. Here, using protein samples from wild type and three STRIPAK mutants, we applied absolute quantification by parallel reaction monitoring (PRM) to analyze phosphorylation site occupancy in SmKIN3 and other septation initiation network (SIN) components, such as CDC7 and DBF2, as well as BUD4, acting downstream of SIN. For SmKIN3, we show that phosphorylation of S668 and S686 is decreased in mutants lacking distinct subunits of STRIPAK, while a third phosphorylation site, S589, was not affected. We constructed SmKIN3 mutants carrying phospho-mimetic and phospho-deficient codons for phosphorylation sites S589, S668 and S686. Investigation of hyphae in a {Delta}SmKin3 strain complemented by the S668 and S686 mutants showed a hyper-septation phenotype, which was absent in the wild type, the {Delta}SmKin3 strain complemented with wild type gene, or the mutant S589. Furthermore, localization studies with SmKIN3 phosphorylation variants and STRIPAK mutants showed that SmKIN3 preferentially localizes at the terminal septa, which is distinctly different from the wild type strains. We conclude that STRIPAK-dependent phosphorylation of SmKIN3 has an impact on controlled septum formation and on the time-dependent localization of SmKIN3 on septa at the hyphal tip. Thus, STRIPAK seems to regulate SmKIN3, as well as DBF2 and BUD4 phosphorylation, affecting septum formation.