Abstract
Intercellular communication of vegetative cells and their subsequent cell fusion is vital for different aspects of growth, fitness, and differentiation of filamentous fungi. Cell fusion between germinating spores is important for early colony establishment, while hyphal fusion in the mature colony facilitates the movement of resources and organelles throughout an established colony. Approximately 50 proteins have been shown to be important for somatic cell-cell communication and fusion in the model filamentous fungus Neurospora crassa. Genetic, biochemical, and microscopic techniques were used to characterize the functions of seven previously poorly characterized cell fusion proteins. HAM-6, HAM-7 and HAM-8 share functional characteristics and are proposed to function in the same signaling network. Our data suggest that these proteins may form a sensor complex at the cell wall/plasma membrane for the MAK-1 cell wall integrity mitogen-activated protein kinase (MAPK) pathway. We also demonstrate that HAM-9, HAM-10, AMPH-1 and WHI-2 have more general functions and are required for normal growth and development. The activation status of the MAK-1 and MAK-2 MAPK pathways are altered in mutants lacking these proteins. We propose that these proteins may function to coordinate the activities of the two MAPK modules with other signaling pathways during cell fusion.
Highlights
Cell-to-cell fusion between vegetative cells plays a critical role in the life cycles of the filamentous fungi
In order to determine whether HAM-6, HAM-7, HAM-8, HAM9, HAM-10, AMPH-1 and WHI-2 influence the activity of MAK1 and MAK-2, we looked at the phosphorylation status of both mitogen-activated protein kinase (MAPK) in mutant germlings during conidial anastomosis tubes (CATs)-inducing conditions (Figure 8)
HAM-7 is a GPI-anchored cell wall protein, and we have previously shown that it functions as a MAK-1 pathway sensor during hyphal fusion
Summary
Cell-to-cell fusion between vegetative cells plays a critical role in the life cycles of the filamentous fungi. In the model filamentous fungus, Neurospora crassa, cell-to-cell fusion plays an important role during colony establishment, as well as during conidiation (asexual development) and protoperithecium formation (sexual development) [5,6,7]. During the N. crassa asexual life cycle, wild type colonies transport nutrients from a vegetative hyphal network into the growing aerial hyphae, which generate conidia (asexual spores). Cell fusion mutants are defective in producing the long aerial hyphae typical of wild type cells. They produce short aerial hyphae, which give a ‘‘flat’’ carpet-like conidiation phenotype. Cell fusion mutants are female sterile, and this may be because the efficient transport of amino acids and other nutrients from a vegetative hyphal network into the developing protoperithecia is needed to support sexual development
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