Evidence against a direct role for cortical actin arrays in saltatory organelle motility in hyphae of the fungus Saprolegnia ferax

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Abstract
Actin stained by rhodamine-labelled phalloidin in growing hyphae of the oomycete fungus Saprolegnia ferax is restricted to an approximately 0.25 μm deep layer of the cell periphery where it forms an apical cap of fine filaments and a subapical array of coarser longitudinal fibrils interspersed with plaques. The functions of this actin are unknown, but because actin-rich fibrils in other cells are involved in organelle motility I have sought evidence for a similar role in these hyphae. The most prominent motile structures are a population of spherical, predominantly sub-micrometre diameter, refractile vesicles of unknown function, which show typical saltatory movements along the hyphae. The motility of these vesicles is statistically identical in both the central cytoplasm, remote from the actin fibrils, and the peripheral cytoplasm adjacent to the fibrils. Treatment of hyphae with specific concentrations and durations of the detergents Tween 20 and Brij 58 causes extensive reorganization of the actin arrays with no effect on vesicle motility, whereas Triton X-100 severely inhibits motility with little detectable effect on the actin filaments. These observations show that normal vesicle motility does not depend on the proximity of a normal population of actin filaments and, therefore, suggest that the latter have some function other than a direct role in saltatory organelle motility.