The STRIPAK signaling complex regulates phosphorylation of GUL1, an RNA-binding protein that shuttles on endosomes

The striatin-interacting phosphatase and kinase (STRIPAK) multi-subunit signaling complex is highly conserved within eukaryotes. In fungi, STRIPAK controls multicellular development, morphogenesis, pathogenicity, and cell-cell recognition, while in humans, certain diseases are related to this signaling complex. To date, phosphorylation and dephosphorylation targets of STRIPAK are still widely unknown in microbial as well as animal systems. Here, we provide an extended global proteome and phosphoproteome study using the wild type as well as STRIPAK single and double deletion mutants from the filamentous fungus Sordaria macrospora. Notably, in the deletion mutants, we identified the differential phosphorylation of 129 proteins, of which 70 phosphorylation sites were previously unknown. Included in the list of STRIPAK targets are eight proteins with RNA recognition motifs (RRMs) including GUL1. Knockout mutants and complemented transformants clearly show that GUL1 affects hyphal growth and sexual development. To assess the role of GUL1 phosphorylation on fungal development, we constructed phospho-mimetic and -deficient mutants of GUL1 residues S180, S216, and S1343. While the S1343 mutants were indistinguishable from wildtype, phospho-deficiency of S180 and S216 resulted in a drastic reduction in hyphal growth and phospho-deficiency of S216 also affects sexual fertility. These results thus suggest that differential phosphorylation of GUL1 regulates developmental processes such as fruiting body maturation and hyphal morphogenesis. Moreover, genetic interaction studies provide strong evidence that GUL1 is not an integral subunit of STRIPAK. Finally, fluorescence microcopy revealed that GUL1 co-localizes with endosomal marker proteins and shuttles on endosomes. Here, we provide a new mechanistic model that explains how STRIPAK-dependent and - independent phosphorylation of GUL1 regulates sexual development and asexual growth. Author SummaryIn eukaryotes, the striatin-interacting phosphatase and kinase (STRIPAK) multi-subunit signaling complex controls a variety of developmental processes, and the lack of single STRIPAK subunits is associated with severe developmental defects and diseases. However, in humans, animals, as well as fungal microbes, the phosphorylation and dephosphorylation targets of STRIPAK are still largely unknown. The filamentous fungus Sordaria macrospora is a well-established model system used to study the function of STRIPAK, since a collection of STRIPAK mutants is experimentally accessible. We previously established an isobaric tag for relative and absolute quantification (iTRAQ)-based proteomic and phosphoproteomic analysis to identify targets of STRIPAK. Here, we investigate mutants that lack one or two STRIPAK subunits. Our analysis resulted in the identification of 129 putative phosphorylation targets of STRIPAK including GUL1, a homolog of the RNA-binding protein SSD1 from yeast. Using fluorescence microscopy, we demonstrate that GUL1 shuttles on endosomes. We also investigated deletion, phospho-mimetic, and -deletion mutants and revealed that GUL1 regulates sexual and asexual development in a phosphorylation-dependent manner. Collectively, our comprehensive genetic and cellular analysis provides new fundamental insights into the mechanism of how GUL1, as a STRIPAK target, controls multiple cellular functions.