Calcium signaling is involved in dynein-dependent microtubule organization

The microtubule cytoskeleton supports cellular morphogenesis and polar growth, but the underlying mechanisms are not understood. In a screen for morphology mutants defective in microtubule organization in the fungus Ustilago maydis, we identified eca1 that encodes a sarcoplasmic/endoplasmic calcium ATPase. Eca1 resides in the endoplasmic reticulum and restores growth of a yeast mutant defective in calcium homeostasis. Deletion of eca1 resulted in elevated cytosolic calcium levels and a severe growth and morphology defect. While F-actin and myosin V distribution is unaffected, Deltaeca1 mutants contain longer and disorganized microtubules that show increased rescue and reduced catastrophe frequencies. Morphology can be restored by inhibition of Ca(2+)/calmodulin-dependent kinases or destabilizing microtubules, indicating that calcium-dependent alterations in dynamic instability are a major cause of the growth defect. Interestingly, dynein mutants show virtually identical changes in microtubule dynamics and dynein-dependent ER motility was drastically decreased in Deltaeca1. This indicates a connection between calcium signaling, dynein, and microtubule organization in morphogenesis of U. maydis.