Summary: The role of actin in apical growth and enzyme secretion in the filamentous fungus was studied by treating the hyphae with cytochalasin A (CA), which inhibits actin polymerization. Indirect immunofluorescence microscopy revealed actin at the tips of main hyphae and branches, and at the sites of developing septa. CA inhibited the growth of the fungus and changed the growth pattern of hyphal tips from cylindrical tubes to spherical beads. The regions with swellings showed no actin fluorescence, and neither was actin seen in association with septa. After 4 h exposure, hyphae were able to resume the normal tip growth pattern in the presence of CA for a short period of time and new cylindrical hyphae, with actin fluorescence at the apex, emerged from the swollen tips. Later, the tips of the hyphae swelled again, which led to a beaded apperance. We also studied the effect of CA on the secretion of α- and β-galactosidase. α-Galactosidase is secreted into the culture medium, whereas β-galactosidase remains in the mycelium, with part of its activity bound to the cell wall. When mycelium was incubated in the presence of CA, a reduction in the secretion of α-galactosidase into the culture medium and a decrease in the α- and β-galactosidase activities bound to the cell wall was detected. However, the CA dose used for the hyphae did not modify the secretion of the enzymes from protoplasts. Results described here provide evidence that a polymerized actin cytoskeleton is required for normal apical growth, hyphal tip shape and polarized enzyme secretion in Cytochalasin-induced disruptions of the actin cytoskeleton could result in the alterations of apical growth and inhibition of enzyme secretion observed by blocking secretory vesicle transport to the apex.


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