The cellular basis of organelle transport in filamentous fungi is still unresolved. Here we have studied the intracellular movement of mitochondria and other organelles in the fungus Neurospora crassa. Four different model systems were employed: hyphae, protoplasts, a cell wallless mutant, and experimentally generated small, flattened cell fragments of the mutant cells. Organelle movements were visualized by DIC optics and computer-enhanced video microscopy. In all cell models the transport of organelles was vectorial and saltatory in nature. The mean velocities for mitochondria, particles and nuclei were 1.4, 2.0, and 0.9 microns/s, respectively. Treatment with 10 microM nocodazole for 30 minutes caused a complete disappearance of microtubules and reversibly blocked directed transport of virtually all organelles, whereas cytochalasin D up to 20 microM was without effect. Correlative video and immunofluorescence microscopy of small fragments of wall-less mutant cells revealed a clear match between microtubule distribution and the tracks of moving organelles. We conclude that organelle movement in the filamentous fungus Neurospora crassa is a microtubule-dependent process.

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