The BEM46-like protein appears to be essential for hyphal development upon ascospore germination in Neurospora crassa and is targeted to the endoplasmic reticulum.

The bud emergence (BEM)46 proteins are evolutionarily conserved members of the alpha/beta-hydrolase super family, but their exact role remains unknown. To better understand the cellular role of BEM46 and its homologs, we used the model organism Neurospora crassa in conjunction with bem46 RNAi, over-expression vectors, and repeat induced point mutation analyzes. We clearly demonstrated that BEM46 is required for cell type-specific hyphal growth, which indicates a role for BEM46 in maintaining polarity. Vegetative hyphae, perithecia, and ascospores developed normally, but hyphae germinating from ascospores exhibited a loss-of-polarity phenotype. We also found that the BEM46 protein is targeted to the perinuclear endoplasmic reticulum (ER) and also localizes at or close to the plasma membrane. Our findings show that BEM46 can be used as a new ER marker for filamentous fungi, the first for N. crassa. Our data suggest that BEM46 plays a role in a signal transduction pathway involved in determining or maintaining cell type-specific polarity. This implies a higher degree of fungal hyphae differentiation than previously expected. This work also has implications for higher eukaryotic cells with polarized growth, such as pollen tubes or neuronal cells.