The role of the fungal STRIPAK complex in hyphal fusion and sexual development

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Striatin-interacting phosphatase and kinase (STRIPAK) complex is conserved in fungi and animals. In the filamentous ascomycete Sordaria macrospora, STRIPAK complex has been found to be essential for hyphal fusion and fruiting-body development. The STRIPAK orthologues in S. macrospora are PRO11 (striatin), PRO22 (STRIP1/2), PRO45 (SLMAP), the serine-threonine phosphatase scaffolding subunit PP2AA, catalytic subunit SmPP2Ac1, and kinase activator SmMOB3. Unlike mammals, fungi comprise only one striatin gene and are therefore good models to study the cellular function of the STRIPAK complex. Recently, we performed PRO11 and SmMOB3 pull-down experiments coupled to liquid chromatography and mass spectrometry (LCMS) and analyzed the LCMS data for putative interaction partners. An uncharacterized protein was identified as putative PRO11 interaction partner with very high confidence, and was named STRIPAK complex interactor 1 (SCI1). SCI1 is an orthologue of small coiled-coil (CC) domain(s) containing proteins found in STRIPAK complexes in yeasts, fruit fly, and mammals. Deletion of sci1 revealed its role in cell-cell fusion and sexual development in S. macrospora, and SCI1 was found to interact and co-localize with other STRIPAK components at the nuclear envelope in vivo. Thus, SCI1 can be considered as a core component of the S. macrospora STRIPAK complex and we proved that our proteomics approach is useful to identify STRIPAK components and effectors. Further detailed analysis of LCMS data from PRO11, SmMOB3, and SCI1 pull-down experiments showed significant enrichment of three proteins of the nuclear pore complex, the activator of germinal center kinases MO25/HYM1, and four proteins involved in phospholipid biosynthesis and transport. These data suggested that the STRIPAK complex may play a role in kinase signaling pathways and in phospholipid signaling. Moreover, nuclear pore complex proteins may act as anchors that link the STRIPAK complex to the nuclear envelope. Alternatively, components of the nuclear pore complex may be regulated by phosphorylation and dephosphorylation events via STRIPAK associated kinases and the PP2A phosphatase. Thus, by studying these new interaction partners of the STRIPAK complex, we aim to understand the role of STRIPAK complex in sexual development and cell fusion in fungi. Beside, these investigations may also help to decipher the function of STRIPAK complexes in higher eukaryotes.