Molecular dissection of SO (SOFT) protein in stress-induced aggregation and cell-to-cell interactive functions in filamentous fungal multicellularity.

Filamentous fungi grow by organizing multicellularity through hyphal compartmentalization and cell fusion. SO (SOFT) protein, which was originally identified in Neurospora crassa, plays distinct functional roles in cell-to-cell interactions, such as septal plugging and cell fusion. We previously reported that AoSO, an Aspergillus oryzae SO homologue, forms aggregates at the septal pore in response to stress, as well as upon hyphal wounding. However, the functional regions that mediate the multicellular functions of AoSO, which is a large protein composed of 1195 amino acids, have not been elucidated. Here, we divided AoSO protein into regions according to amino acid sequence conservation among other fungal SO homologues. By heterologous expression of full-length and truncated forms of AoSO in the yeast Saccharomyces cerevisiae, the region responsible for the stress-induced aggregation of AoSO was identified to be between amino acids 556 and 1146. In A. oryzae, however, septal localization of AoSO aggregates required the 49 C-terminal amino acids. Thus, expression of only the C-terminal half of AoSO was sufficient for septal plugging and prevention of excessive cytoplasmic loss upon hyphal wounding. In contrast, the N-terminal half of AoSO, from amino acids 1 to 555, together with the C-terminal end, was revealed to be indispensable for cell fusion. Collectively, these findings suggest that the C-terminal half of AoSO, which mediates stress-induced aggregation, is required for both septal plugging and cell fusion, whereas the N-terminal half confers an additional functionality that is essential for cell fusion.