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Fungal hyphae and plant pollen tubes are among the most highly polarized cells known and pose extraordinary requirements on their cell polarity machinery. Cellular morphogenesis is driven through the phospholipid-dependent organization at the apical plasma membrane. We characterized the contribution of phosphoinositides (PIs) in hyphal growth of the filamentous ascomycete  Neurospora crassa .  MSS-4  is an essential gene and its deletion resulted in spherically growing cells that ultimately lyse. Two conditional  mss-4- mutants exhibited altered hyphal morphology and aberrant branching at restrictive conditions that were complemented by expression of wild type  MSS-4 . Recombinant MSS-4 was characterized as a phosphatidylinositolmonophosphate-kinase phosphorylating phosphatidylinositol 4-phosphate (PtdIns4P) to phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P 2 ). PtdIns3P was also used as a substrate. Sequencing of two conditional  mss-4  alleles identified a single substitution of a highly conserved Y750 to N. The biochemical characterization of recombinant protein variants revealed Y750 as critical for PI4P 5-kinase activity of MSS-4 and of plant PI4P 5-kinases. The conditional growth defects of  mss-4  mutants were caused by severely reduced activity of MSS-4(Y750N), enabling the formation of only trace amounts of PtdIns(4,5)P 2 . In  N. crassa  hyphae, PtdIns(4,5)P 2  localized predominantly in the plasma membrane of hyphae and along septa. Fluorescence-tagged MSS-4 formed a subapical collar at hyphal tips, localized to constricting septa and accumulated at contact points of fusing  N. crassa  germlings, indicating MSS-4 is responsible for the formation of relevant pools of PtdIns(4,5)P 2  that control polar and directional growth and septation.  N. crassa  MSS-4 differs from yeast, plant and mammalian PI4P 5-kinases by containing additional protein domains. The N-terminal domain of  N. crassa  MSS-4 was required for correct membrane association. The data presented for  N. crassa  MSS-4 and its roles in hyphal growth are discussed with a comparative perspective on PI-control of polar tip growth in different organismic kingdoms.

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The Essential Phosphoinositide Kinase MSS-4 Is Required for Polar Hyphal Morphogenesis, Localizing to Sites of Growth and Cell Fusion in Neurospora crassa