BRO-1 interacts with the SO protein and is essential for germling communication and fusion in Neurospora crassa

Colony initiation of filamentous fungi commonly involves fusion of germinating vegetative spores. Studies in Neurospora crassa revealed an unusual cell-cell communication mechanism mediating this process, in which the fusion partners coordinately alternate between two physiological stages, probably related to signal sending and receiving. This “cell dialog” involves the alternating, oscillatory recruitment of the SO protein and the MAK-2 MAP kinase module to the apical plasma membrane of growing fusion tips.

We identified BRO-1, the homolog of the mammalians ALG-2-interacting protein X (ALIX), as a new factor participating in germling interaction and fusion. Alix is an ESCRT accessory component and has a main role in regulating the biogenesis of exosomes and the secretion of extracellular vesicles. In N.crassa,BRO-1 is essential. Reduced expression of the bro-1 gene results in a ∆so-like phenotype, resulting in the loss of chemotropic interactions and subsequent fusion. Subcellular localization and live cell imaging revealed that BRO-1 is recruited to the tips of the interacting germlings in a dynamic, oscillating manner, such that high signal intensity of BRO-1 in one interacting tip correlates with low signal intensity of BRO-1 at the interacting tip of the fusion partner. Co-localization experiments of BRO-1 and SO revealed that both proteins co-localize in the described dynamic manner at the growing, interacting cell tips.

We hypothesize that BRO-1 plays a role in signal secretion during cell – cell communication. Future analysis of its molecular function will greatly contribute to our understanding of the unique “cell dialog” mechanism and the molecular bases of fungal cellular communication.